Oklahoma Transmission Supply is the best place I found for parts when I rebuilt mine. They beat RockAuto's prices, and they carry all the Sonnax bits as well as Raybestos high friction clutches.
www.otsparts.com
Pay very close attention to the direction the seals are installed when you perform your teardown, and make lots of notes! For all my searching, I couldn't find references that say which direction the seals go, and it's critical.
There are lots of crappy "700R4 Rebuild" books out there that only have 10% of the information you need.
"Rebuild" kits only have about 10% of the parts you need. I bought parts about five different times as I found more stuff broken, and having to buy something on a separate order cranks up the RockAuto costs. I paid about $700 total for my parts, and if I'd just bought once from OTS (I can drive to their store), I probably would have only spent about $550. This was a couple years ago, so your inflation may vary.
Here's what I ended up buying:
Rebuild Kit - Pioneer 750033 - $40.89
High Friction Clutches - Raybestos RCP96087 - $35.79
Steels Assorted - $44.47
Line Pressure Booster Kit - Sonnax 700R4-LB1 - $14.88
Extra-Wide Kevlar Band - Sonnax 77700-01K - $29.79
Hardened Sun Shell - Sonnax 77749-02K - $80.79
2nd Gear “Corvette” Servo Kit - Sonnax 77701-04K - $24.79
Super-Hold Servo Kit Sonnax - 77767K - $50.89
5-Planet Reaction Carrier - AC Delco 24241244 - $230.99
Overrun Clutch Spring Plate - GM Genuine 24206085 - $5.20
Case Vent Tube - GM Genuine 24202440 - $5.47
Filter - Raybestos 515700 - $14.13
Oil Pan - GM Genuine 8667545 - $17.48
Valve Body Separator Plate - $16?
Bushing Kit - $17
Line pressure valve c-clip - $1?