For both Superformance and Shelby CSX, check out the offerings on
427 Cobra Country--Ford AC Cobra replica manufacturers SUPER-SITE.
Do your homework of course (that means, get an inspection or have a complete history from a reputable dealer/builder, avoid "home grown" builds) but most Cobras are weekend toys and don't get driven much so used is usually not an issue. The plus side of a used one is that it will be "sorted out" (unless someone gave up). Every Cobra will have its nuances, and if your builder is 500 miles away that can be a logistics issue.
Good luck