Before you start looking for a provider, you should stop for a moment and consider what exactly you need. How many transactions do you expect to be completing in a month? How many products do you have to put on your web site? How complex does the software need to be? How much are you willing to spend?
There are a lot of online transaction providers out there, and they all have varying packages. Deciding on a provider's package that fits your needs is perhaps the most important aspect in creating an eCommerce website.
The Directory page provides a good cross section of the companies out there. It is a good idea to go through a few of them and compare their prices and services. For a quick breakdown of the most popular packages these companies provide, visit the Summary section.