MudPie, you're talking about the freehub body, right?
If you're reasonably handy and have tools, I understand that it's a pretty simple matter to replace the freehub body (the part of the hub with splines that you slide the cassette on to). Your LBS may not have one in stock, but they might be able to order one for you. If not, Harris Cyclery has Shimano freehub bodies, I think.
If you have a freewheel, you can replace the freewheel. New ones are generally pretty cheap - I paid about 20 bucks for a new 13-34 seven-speed freewheel. If you don't already have one, you'll need a freewheel remover that matches the freewheel, and a big enough wrench to fit it. The Park Tool chainwhip has a hole in the end of the handle that fits their freewheel removers/lockring tools, so you may be able to convince your LBS mechanics to let you borrow their Park chainwhip for two minutes if you don't have one of your own or a big enough wrench.