I am looking for a PRC that does what the Dragon Disciple PRC does but with Celestials instead. I do not own the Book of Exalted Deeds so I figured if anything would have something like it that would be it.
No, have an Assimar player that has taken Assimar Racial feats from Races of Fareun, and is joining a secret society of Celestial decended folks trying to fight the good fight.
I use a home ruled variant of the Assimar race which removes the native outsider tag from them. A celestial apothesis seemed allowing the Native Outsider tags seems a natural fit.
I am looking for a PRC that does what the Dragon Disciple PRC does but with Celestials instead. I do not own the Book of Exalted Deeds so I figured if anything would have something like it that would be it.
Something vaguely similar may be the Celestial Mystic from the BoED, p. 55. Must be a spellcaster and have taken two Vows (feats) among other stuff. The CM gets Damage Reduction, Energy Resistance, Celestial Spells, some sort of Spell Resistance, can Cure her own wounds, and get gets some kind of Bardic Knowledge, amongst other things. It's a 9/10 spellcaster PrC. But there's no such thing as "Celestial Apotheosis" at 10th level.
By the way: What about the feat Celestial Heritage from the Planar Handbook, p. 38?
Added: You may go the way of the Divine Agent from the Manual of the Planes [Remember, there's a 3.5 Update from WotC for the Divine Agent). Must be able to cast 2nd-level divine spells (some other requirements, too). At 9th level she becomes an outsider. She gets Godly Gifts and Altered Appearance and other more useful goodies. It's a 5/10 spellcaster PrC.
You could always change the details of the Dragon Disciple so that it becomes a "Celestial Disciple" instead. Perhaps instead of picking a Dragon Colour, you choose a Celestial type (Archon, Eladrin, etc..) and slowly gain the half-celestial template.
It requires DM/Player cooperation of course but it saves looking through books you don't have access too.
If you mean that they will take on the 1/2 celestial template at the end of the class, you may have to do that yourself. Just remember, the 1/2 celestial template is +4 LA, more than the half-dragon. So you may have to have them lose the 'bonus spells' feature of dragon disciple, and just spread the stat boosts, spell-like abilities, resistances, and flight over 10 levels.
The scarred lands wizard/sorcerer/bard splatbook has something like this. I haven't read it recently, but I don't remember it being as big a change as the dragon disciple. Of course, you could always use the creature levels from Savage Species, just pick an appropriate celestial to work towards.