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)