I've written before about the usertesting.com service, which I've used and I like. Here's a scenario and example task script written to recruit participants to try out a shopping cart.
There seem to be tons of similar services available now. I'm sure many of them are great. I just found one that worked for me and haven't had a reason to look elsewhere. Plus they include things like this on their blog.
Improving your shopping cart - usertesting.com