I recently wanted to upgrade my Mercury accelerator which uses up to 8 30pin SIMMs from 16MB to 32MB and I had a very tough time finding a source for the 4MBytes 30pin SIMMs required by this accelerator. Not a single Amiga dealer that I know of had them.
1MBytes 30pin SIMMs are easy to find but 4MBytes are much more difficult to find.
I know the Fusion-Forty also requires 30pin SIMMs and loading the accelerator to its maximum of 32MBytes of RAM also requires 8 30pin SIMMs of 4Mbytes each.
I found what I was looking for here:
http://www.memory.com/item.asp?item=TS4M8260Service was fast and these were perfectly new SIMMs. The newer 2 chip configuration consumes much less power than the old 8 or 9 chips versions of the same type of SIMMs.
They also sell 72 pin SIMMs that are compatible with Amiga accelerators but the pricing seems similar to what Amiga dealers ask for.