Look in the classifieds here, under "parts". A guy in the midwest is parting out his 5.0, he shows a base for sale. I'm pretty sure the 5.0, 5.2 & 5.7 all use the same base...er I just re read your post...are you looking for older Nacra?
-- Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap --
Is performance cat not making them anymore? There are a lot of boats out there with that base! Hopefully they will still manufacture the mast balls!
New style base is a bit of a misnomer, I believe they switched from the 1.25" open ball system to the 1.5" captive system around 1985. The 5.0, 5.2, 5.5, 5.7 and 5.8 all use the same base and mast extrusion (maybe even the 6.0).
Many Nacra mast bases have been damaged by lowering the mast without rotating it. This generally breaks the tongue where the stepping pin goes, but the base is often still usable. A good machine shop can fix the tongue (especially if the broken piece is still available) or you can step the mast without it (a bit more dangerous but not bad if you have 3 people). Check out the classifieds, if in doubt give Dan Berger a PM he may have some broken ones you can buy cheap until you find a whole one.