The Dragon #321 article "Creatures of Brilliance: Inhabitants of the Plane of Brilliance" details the template radiant (73-75). The template increases a creature's CR by but 1, but, having used this template before against PCs I can say that value is likely underestimated. (In particular, I found HD 13 too low for even a once-per-day use of, as a spell-like ability, prismatic spray.)
However, although it teeters on the edge of official-dom (i.e. few DMs allow the material despite it being approved by Wizards of the Coast), Athas.org released in 2009 Terrors of Athas containing the template sun incarnation (96). The template says that
A sun elemental incarnation has a body of liquid fire and a fierce solar aura. Its claws or teeth are scorched bone, its hair or fur wisps of trailing smoke.
Mechanically, the template affects a creature's Challenge Rating identically to the template celestial, so if maintaining a creature's CR is the primary concern, this works. Note that while the template celestial can be applied to "any corporeal aberration, animal, dragon, fey, giant, humanoid, magical beast, monstrous humanoid, plant, or vermin of good or neutral alignment," the template sun incarnation is limited "to any corporeal aberration, animal, magical beast, plant, or vermin," but the sun incarnation template has no alignment restrictions.
Likewise teetering is the 2005 Planewalker.com release Denizens of the Inner Planes (Planewalker.com is still recovering from a database crash; a site mirror still hosting this document is here). That text contains the CR 3 radiant mephit (70) and the CR 1 through 11 (depending on size) positive quasielemental radiance elemental (97-100). However, of more interest might be Part 3: Planar Encounters (139-48), which lists creatures from a variety of official Wizards of the Coast sources (as well as itself) that are appropriate to the various elemental, quasielemental, and paraelemental planes, some of which might provide inspiration for a sun-themed summoner. (Sadly, these lists appear to stop with Monster Manual III, so you're digging through IV and V on your own.)
More expansively, ORyan's RPG Locker hosts "Monsters and Gods Sorted by Plane," a labor-of-love name-and-book list of where extraplanar monsters reside that spans Dungeons and Dragons from its beginning through 3.5. It's a jaw-dropping—maybe overwhelming—resource for the budding D&D planar cosmologist and should provide plenty of inspiration.