A couple of stores here in the Seattle area typically have the more rare Plecos; The Fish Store and A Place for Pets. I believe they get them from The Wet Spot (Retailer/Wholesaler) in Portland.....you might start there.
Did you make it to Seattle for the GSAS meeting last month, we had Barbie Fiorentino as our guest speaker? She is a Pleco breeder (and LFS owner) of national (and international) renown from Spokane. I am not into Plecos, but she did a great talk which is available for members to download from our GSAS website. You can join online with Paypal.