I'm in the same zone you are, and there are lots that have done well for me, but in thinking about it, many are more restrained in color than you're looking for. Some of my wilder colored ones are Josephine and Dr. Rupple. The texensis clemmies have done well for me and are mostly reddish and an unusual tulip shape. In general, for me, the type III pruning ones are the easiest and bloom heavily here. There are some new ones I have that are also unusually shaped like Rooguchi, which I like a lot but does tend to mildew a little. I think my hands-down favorite of all I grow is Rhapsody. For me it is an intense sapphire blue - my favorite color of all time. I can't stop looking at it when it's in bloom. I have mine planted under a crabapple tree and I just read somewhere that it may be more purple than blue in fuller sun, and mine is fairly young, so it's not a blooming machine, but the color is unbelievable.
As far as where to buy them, I've rarely had much luck with local nurseries which sell the more common kind. I've bought a lot mail order, with good success, particularly if I pot them up for a year or so. Hope this helps a little.

Look at gardencrossings.com as they have several kinds. I ordered the Crystal Fountain clem from them and I was VERY impressed with the way it was packed. It arrived in great shape and it is very healthy. They are a little pricer but it was worth it.

Thanks, opheliathornvt and hardin! Wow, gorgeous clematis-I appreciate the info on gardencrossings!
:0) Phyl