I have never heard that there is a PowerPC 970GX processor in existence, yet this article by IBM says:
I remember some rumors about this GX, but this seems to claim there is one. The article was published Mar. 1. 2005.
The only processors currently supporting AltiVec are the G4 and G5. The G4 (including model numbers 7400 and 7410) and G4+ (7450 and 7455) processors are made by Motorola. (There are more models than just the ones listed here, but these are the most widely discussed.) The G5 chips include the IBM 970, 970FX, and 970GX; these are essentially POWER4 cores with an AltiVec unit bolted on. So far, only PowerPC® processors have had AltiVec support, not the POWER line. If you want to buy "a computer with AltiVec," Apple's Mac line is your most likely option. For evaluation boards and custom designs, however, you can go with any of the many vendors who do development kits based on either the G4 or G5.
I remember some rumors about this GX, but this seems to claim there is one. The article was published Mar. 1. 2005.