My experience on the POR-15 is that the best deals are on-line, even if you have to pay shipping. You can find it on ebay and Amazon, or at Vintage Trailer Supply. You might look for automotive specialty paint stores in your area. I have found it at AJ Foyt, which I think is a chain.
I've had a heck of a time finding wheel wells for single axle trailers. You can order some from vintagetrailering.com, but they seem kind of pricey to me. I tried to source some locally (Houston), and was looking at about $300 for the pair (which still feels pricey).