I'd looked into this issue a few times over the last few years. The largest dedicated FIFO's you can find these days are 256Kx36. You usually pay through the nose for them and they usually have excessive lead times. They are manufactured by IDT with generic part number 72T36125.
We settled on implementing FIFOs with an FPGA, an SDRAM, SDRAM controller IP, and a bit of custom logic. With this setup, considering what parts are commonly available with short lead times, we wound up with a 512Mx32 FIFO, which was way beyond what we were orginally looking for.
The memory situation has gotten to the point where the small specialty FIFOs are very expensive and generalized memory is really, really big, but cheap as borscht.
Dwayne Surdu-Miller