3rd Party Installation/Customization
Currently, there are a small handful of cart installers. Below is the list of people who can implement your cart with your existing store, or create a new one for you.

Nebulus Designs
Daryl Isaacs.com*
IrelandNow.com*
G&A Marketing

*These installers also provide hosting

Customization

Of course, most customization can be handled through the forum, but should a specialized customization on the cart or back-end be needed for transaction capturing, Nebulus Designs authored NebuCart so can handle any special modifications you may need.

They specialize in JavaScript (of course), graphic design, ASP and databasing, as well as many other web technologies. Want to integrate Flash with your NebuCart? It can be done! Check out their site for a full skillset listing.