As I've recently discovered, 8spd cassettes come in two varieties. The one where each cog can slide seperately onto the freehub and the one where the last 3-4 cogs are permenantly mated to a spider which slides onto the freehub.
If you're ordering one from eBays venders you need to ask that question.
I'm unsure what the deal is with the 8spd cassettes and Harris Cyclerys statement of them no longer being 'customizeable' .
The width of cogs for the 7spd cassettes and the 8 spd are pretty much the same. Use the 8spd spacers (Loose Screws sells em)
I just put togeather a nice 13 15 17 19 21 23 25 28 setup that has even spacing thruout. Bought the 12-25 on eBay and used a 28t 7spd cog to round it out. (substituted the 28 for the 12)
I dislike the large jump so many mnfg. 7/8spd cassettes offer in the teeth between a 6th cog and the 8th.
Universal Cycles offers a selection of 8spd cassettes, Harris and Loose Screws do as well.
If you find you'd like to grind the heads off the 3 rivets holding the cassettes together and play with cog changes many of your LBS have a ton of 'old stuff' in shop back rooms and basements.