They use 72 pin SIMMs...pretty easy to find, and cheap. Commonly found in later 486 and early Pentium PCs, as well as the following Macs:

LCIII/Performa 450

LC475/Performa 475/476/477/Quadra 605

LC/Performa 520

Performa 550

LC/Performa 575, 576, 577, 578

LC/Performa 580, 588

Mac TV

Quadra/Centris 610

Quadra/Centris 650

Quadra 800

Quadra 840AV

Power Mac x100

Power Mac/Performa 5xxx/6xxx (apart from 5400, 5500, 6360, 6400, 6500)