Try equine.com or equinenow.com. It's definitely harder to find a tall WB between those ages under $5K, but it can be done. I've found that sometimes taller horses can intimidate riders and get away with bad behaviour then they are sold as problem horses or mature horses that need to be restarted. There's deals out there right now for a confident, experienced rider willing to put in a little effort.
My sister just bought a 17.2 Imported (from Germany) 9 year old Oldenburg (she copied me, lol) for $4000. He vetted clean but he was for sale because he bucked his rider off and then was left to pasture. He apparently has great movement but was too slow at Spruce Meadows for upper level competition though.