First, the obvious:
the Official Ampeg Page, for checking out what Ampeg is up to now.
Vintage Amp Restoration is the best all-around source for tolex, grill cloth, and hardware for Ampeg amplifiers. They are highly thought of for their speaker reconing, and they make amp cabinets and covers as well. They also carry certain Ampeg transformers, including for Portaflexes, which are again being wound by the original manufacturer and even carry the original part number.
Another excellent source of Ampeg parts is found at Fliptops.com. The number of available parts, both NOS and reproduction, seems to increase each time I visit the site. They even have repro tube cages last I checked, and a selection of NOS transformers. They are truly dedicated to keeping those old Ampegs warm and alive.
Johnson's Extremely Strange Musical Instrument Company carries the extra-extra-long scale strings for Ampeg horizontal basses, and they might have (or be able to create) a part that's missing from your Ampeg bass. Also, now available are prototypes of the new Ampeg scroll basses that Bruce Johnson had designed for the Ampeg Company. Not simply reissues, but reengineered, totally modern instruments, they nonetheless preserve that cool Ampeg vibe. A must-read is Bruce's section on horizontal bass set-up and adjustment, which will help you get the best sound from your bass.
Angela Instruments has tubes, capacitors, etc. and might have an old transformer lying about. They also carry new transformers, and one might fit your need.
Weber VST is a manufacturer of Jensen speaker clones and other amp speakers. No relation to Gerald Weber.
Mojo carries speakers and other parts. If they have something you need, you'll have to find a Mojo dealer, since they don't sell direct to the public.
Parts is Parts carries some Ampeg grill cloth and tolex.
Sovtek carries many tubes, and now other parts, of use to Ampeg owners.
Vibroworld carries many parts, including some of interest to Ampeg afficionados. There is also some wonderfully instructive material on the difference between tremolo and vibrato.
Triode Electronics is another source for tubes and related parts.
Subway Guitars might have some Ampeg instruments in their ever-changing stock.
Joe Piazza is kind enough to have sent me most of the Ampeg schematics on the Schematics page. Check out his home page as well, which now includes voltage charts for many of the schematics he has provided.