turtle,
don't sweat the TPI too much. schwinn forks all used 24tpi, just like most of the world did. The Brits used 24tpi on most makes, but old Raleighs will have a 26tpi fork/headset. The French and the Swiss had their own standards, too. But, bottom line: balloon bike cruiser forks will have a 24tpi, and you can find them all day. (You're probably thinking about the american BB for OPC, which schwinn used a 28tpi but almost everyone else used the more common 24tpi. )
The details you WILL want to keep in mind: first and foremost, how long you need the steerer tube to be. It'll probably be around 6" (160mm) for your old Schwinn, but that's just my educated guess. *Measure to make sure*. A little bit too long is ok; you can use spacers. A little bit too short is barely workable, if you can find a suitable headset with lower stack height, but youprobably can't, so don't go a little bit shorter. Too short is entirely unworkable, without welding on a new steerer tube. Also, whatever fork you get, make sure there is a long enough threaded section. (No big deal; there probably will be.)
Another detail: the space between the dropouts. If you were trying to spread the fork that broke, you probably have a 100mm O.L.D. hub and a fork designed for 91mm hubs. Measure your hub before you buy anything, and make sure you get a fork with the proper spread.
Final thing to keep in mind: a fork will have a little "seat" upon which the crown race is pressed. Some of these measure 26.4mm, others measure 27.0mm. Most old bikes, esp old US-made schwinns, have a 26.4 race seat. A lot of new, China/Taiwan-made replacement forks take a 27.0 race. This isn't a terribly big deal, but you'll need to have the right sized crown race in order for it to work. A 27.0 race on a 26.4 seat will not steer reliably (slip), and you won't be able to fit a 26.4 race on a 27.0 seat without grinding it down. They sell crown races for loose-ball headsets cheeep, but you'll wanna know about this bugaboo before you try to put it all together and find out it's not workable without more parts/work. (All this info applies to 1" forks only; newer/bigger forks tend take bigger races, and they've pretty much universalized the sizing for 1 1/8" forks.)
All that being said, I have some 26" forks that I'm going to be listing for sale soon, and all but one of them is gonna be really cheeeep. Measure all yer stuff first, and shoot me a PM. Maybe we can work something out....
HTH,
-Rob
