On the IIfx and the IIfx Reloaded, the two Peripheral Interface Chips use a 65C02 core running at 4MHz - they require SRAM to operate.
Here's a quick list of pin compatible 32K x 8 SRAM chips.
Manufacturer
Part Number
Lyontek
LY62256
Sony
CXK58257AP
Toshiba
TC55257BFL
NEC...