You're fortunate in that Githyanki lore has remained remarkably consistent across the various editions. Even their 4e lore, which shifted them to an entirely new cosmology, preserved their lore in broadstrokes.
The oldest lore source you can find is probably the original "A Guide to the Astral Plane" for AD&D 2nd edition and part of the Planescape line. This is where the largest amount of lore outside of that invented for the "Planescape Monstrous Compendium Appendix 1" is found, which itself was building upon the very limited lore first presented when they debuted in the "Fiend Folio" for AD&D 1e.
In 3rd edition, githyanki lore can be found in the Monster Manual, the Expanded Psionics Handbook and the Manual of the Planes (3.5 edition). 3e is also home to what you might find the most useful, in the "Incursion" adventure path/mini-campaign setting. To quote from 1d4chan:
Dragon #309 contains the core of the Incursion content, with roughly half the issue dedicated to it. It contains details on possible reasons behind the incursion, an examination of githyanki tactics, statistics for their war machines, advice on incorporating the incursion into your campaign (with particular attention on how the generic invasion plan would be modified if the githyanki attack Faerun or Oerth), brief discussion on how to replace the githyanki with other invading races, instructions on running the incursion campaign at low, mid and high levels, adventure seeds for all three tiers, githyanki encounters for all three tiers of play, and a tactical guide to fighting githyanki both directly and in an army.
Dungeon #100 contains the high-level planar adventure "The Lich-Queen's Beloved", which is set up as a potential end-game (or near-abouts) module for the Incursion campaign, with the party seeking to either turn the tide of the war by assassinating Vlaakith CLVII or to cement their victory by killing the lich-queen who started the incursion. Of course, one could also run it as a standard planar adventure, or even the climax of a rebellious githyanki campaign. It features stats for Vlaakith and her iconic artifacts, the Crown of Corruption and the Scepter of Ephelomon, an explanation of what happens to Tu'narath depending on whether Vlaakith is killed or she lives to attain godhood, stats for the githyanki undead of Kr'y'izoth and Tl'a'ikith, and three new magic items; the Coldfire Candle, the Eye Tyrant Armor, and the Slaad Cloak.
Polyhedron #159 contains a brief history of the Githyanki and a guide to their culture and unique gear, stats for two of their monstrous minions - the b'kallash dreadnought and the spectral hound, playable (non-psionic) stats for both githyanki and duthka'gith characters, githyanki prestige classes, and The Invasion of Pharagos; lore for a specific form of the Incursion campaign from Dragon #309 in which the githyanki are after the corpse of the former patron goddess of their ancestors, which can either be used to run a standard Incursion campaign, or to run "Knights of the Lich-Queen", an inverted Incursion campaign in which the players take the role of loyal githyanki and duthka'gith.
In 4th edition, githyanki are fleshed out in the splatbook "The Plane Above: Secrets of the Astral Sea" and in the article "Tu'narath: City of Death" in Dragon Magazine #377.
Githzerai on the other hand, have a much more complex history.
Like githyanki, they debuted as enemies in AD&D 1e's "Fiend Folio". Then they got expanded in 2e. They debuted as a playable race in the Planescape Campaign Setting boxed set, alongside the now-famous Tieflings and the now-obscure Bariaur (bighorn sheep centaurs from the Valhalla-esque Viking-themed plane of Ysgard). They get some slight PC-focused expansion in the Planewalker's Handbook, but the bulk of their new info is to be found in the Planescape Monstrous Compendium Appendix 1 and the Planes of Chaos boxed set. If you're new to D&D and were introduced by 5th edition, you'll find these early githzerai quite surprising, in that they are largely defined as a) devoted anarchists, to the point "you cannot have a Lawful alignment" is literally one of their player-focused weaknesses, and b) knock-offs of the githyanki, with emphasis on their Vlaakith-esque (but less competent) tyrannical mage-king and their "zerths"; a caste of fighter/mages who acted as priests for an underground religion devoted to a githzerai hero named Zerthimon.
In 3rd edition, the githzerai shifted to the more Lawful-aligned "Monk-centric" race you'll know them as from 5th edition. This was inspired by Dak'kon, a zerth party member from the cult-classic videogame Planescape: Torment, whose entire thing was that he was very, very different to the standard gith of AD&D. Gith in this edition can, like the githyanki, be found in the Monster Manual, Expanded Psionics Handbook, and Manual of the Planes.
4th edition saw githzerai change again, if relatively slightly. They first appeared in the Monster Manual, and were then promoted to a playable race in the Player's Handbook 3. They also appeared in "The Plane Below: Secrets of the Elemental Chaos" and "Heroes of the Elemental Chaos", a more PC-focused counterpart to the latter, and had a dedicated article in Dragon #378's "Playing Githzerai". Although 4e moved them from Limbo to the Elemental Chaos as part of its new cosmology, the biggest change was the introduction of the idea that there are githzerai communities in the mortal world ("Prime Material Plane", in 5e terminology), something that goes a long way towards making them more justifiable as a PC race in non-planar campaigns. This element is mostly discussed in the aforementioned "Playing Githzerai".