First, note the free spells a wizard adds to their spellbook every level do not involve any of the spellbook rules—they are automatic, free, and require neither a source nor any skill check. Collegiate Wizard from Complete Arcane can double the number of those, too, if you like; that can be very valuable in a campaign where you will not often have access to sources to copy spells from, though your typical jade phoenix mage will not need it.
Jade phoenix mage requires that you already be 6th level before you can start taking the class, so you should have 9 ranks already. At that level, a wizard ought to have a minimum of 18 Intelligence (corresponding to a starting score of 16 and a headband of intellect +2). Between those, you have a +12 bonus, or check of 22 when taking 10, as you always would copying spells.
Since the DC to do so is 15 + spell level, a check of 22 is sufficient to copy all but 8th- and 9th-level spells. Meanwhile, jade phoenix mage also misses a couple of levels of spellcasting, so when you take your final level of it at 16th, you still have, at best, the spellcasting of a 14th-level wizard—so no 8th- or 9th-level spells for you anyway. For your next level, you won’t be taking jade phoenix mage—you already have all of those levels—and whatever you do take, if it progresses spellcasting, almost-certainly provides Spellcraft in-class. So one rank at 17th gets you the check of 23 you would need for your new 8th-level spells, and another rank at 18th or 19th will get you 24 for 9th-level spells.
And really, your Intelligence should be better than that, and you should have a headband of intellect +6 at that point, so really you won’t even need those ranks.
So as a jade phoenix mage, you should already have all the Spellcraft ranks you will ever need for the sake of copying spells into your spellbook.
Barring that, there are several feats that will allow you to continue gaining ranks in the skill:
If human or doppelganger, including all various half-human races per a sidebar in the same source, Able Learner allows you to get cross-class skill ranks for one skill point apiece instead of two. Races of Destiny.
If you are an elf from Aerenal, or if your DM adapts the feat to your campaign, Aereni Focus can get you any one skill permanently in-class. Player’s Guide to Eberron.
If you are Chaotic, Flexible Mind can get you two skills of your choice permanently in-class. Dragon vol. 326.
If you are from the Sasserine Noble District, or your DM adapts the feat to your campaign, Academy Graduate can get you three Charisma- or Intelligence-based skills permanently in-class. Savage Tide Player’s Guide.1
- Note this is the 3.5e edition of the adventure. I believe it was remade for Pathfinder; no idea if that one has the same feat, not that it matters here.
Keeper of Forbidden Lore gets you Knowledge (the planes) and Spellcraft permanently in-class. Note that it doesn’t require anything of your character beyond Intelligence (13, to be precise), but taking more than one Abyssal Heritor feat forces your alignment to Chaotic unless you also take the Ordered Chaos feat. Fiendish Codex I.