Am I correct in assuming that with a 8 speed cassette will fit on a 10 speed rear hub, and that I will need some spacers?

No spacers required for 8 or 9.
The 10 speed does require a thin spacer.

Wow ,that was quick!!!! Thanks

Yes with a couple of specific exceptions. Shimano's first 10-speed Dura Ace hubs (7800?) and their contemporary Dura Ace and Ultegra prebuilt wheels used an aluminum freehub body with taller splines that accept ONLY 10-speed Shimano cassettes.

Later Dura Ace (7850, 7900, etc.) and all other 10-speed freehubs will take 8, 9 or 10-speed cassettes. The 8 and 9-speed cassettes need no spacers and 10-speed cassettes all come with a 1 mm spacer that goes behind the largest cog.

I just recently purchased a serotta colorado, and i dont know what kind of cassette it has on it. I want to buy a new cassette. The model is anywhere from 1999-2001. Can anybody please help??

PS: the guy at my local REI told me that it might be unavailable to buy in stores??

If I might add another exception... Not every drivetrain out there is Shimano. In the less-likely event we're talking about Campagnolo gear, 8-speed cassettes are incompatible with 9/10/11 speed hubs. Likewise, 8-speed hubs are incompatible with 9/10/11 speed cassettes.

You'll need to buy a cassette that matches the rest of your drivetrain components. What brand are your shifters and derailleurs? How many cogs (gears) are there on the rear wheel?

If you're planning to buy a cassette with a larger big cog, you may need to determine if your rear derailleur can handle it. In that case, the model of the derailleur comes into play, along with tooth counts of the cassette and your crankset.

How many cogs and is there any identifying information written on the lockring? That should tell you the make and type. Your REI guy should have known better unless it was a common type and make of cassette but a rare configuration and that's what he meant.

Took the bike back in and figured evrything out. Thanks!