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.
75 Gallon, 2X55W AH Supply CF 8800K, 1XMarineland Doublebright, 2X Marineland 350 Magnum; 45 Gallon Tall, 96Watt AH Supply CF 6700K; 30 Gallon Long; 2X36W AH Supply CF 6700K; 20 Gallon, 1X26W AH Supply LED kit; all with Press. CO2; Heat Treated (Calcined) Montmorillonite Clay
Greater Seattle Aquarium Society (GSAS)