It was recently asserted here that TTL parts aren't hard to find, but I'm having a heck of a time finding the 74S01 and 74S73. As far as I can tell from the data sheets, the 74S103 is functionally and pin-compatible with the 74S73, but a little faster. I wouldn't mind substituting that, but it doesn't seem any easier to find. The 74S03 and 74S107 are non-pin-compatible replacements for the 74S01 and 74S73, but I'd much prefer to stick with pin-compatible parts. I'm also looking for the 2518 and 2533 shift registers. The 2518 is a hex 32-bit static shift register, and the 2533 is a 1024-bit static (not dynamic!) shift register. The original parts were Signetics, but I don't mind using other sources: Signetics 2518 2533 TI TMS3122 TMS3133 Fairchild 3349 3533 National - MM5058 GI - 2533 AMD - Am2833 Eric