I've heard good things about Fringe Sports Equipment. And they have a Prowler for $159 USD with
free shipping. I would definitely look into that.
Other than that, the usual players such as Rogue and EliteFTS will have high quality but expensive sleds and Prowlers as well.
You could also turn one of those tires you have into a sled to pull around. That's what I do and it works great for conditioning legs and it gets your lungs moving.