Any ideas on where I can find a radiator for my '64 Valiant 225?

Don't fret that you can't get factory correct one. Radiators are one of the easier parts to adapt, unless you want a fan shroud too.

My 64 Valiant 225 had a non-factory radiator when I bought it. The core is ~22" W. Could be non-Mopar. I'll try to post a photo when I get home. The stand-off brackets set the core ~1.5" off the front so the 19" opening doesn't mask the core much. No w.p. fan, just an pusher electric fan from a Mercedes. My radiator is likely for a ~72+ slant (26" wide) since the right bracket has a wide block-off plate.

Your slant has 1.5"D tubes on top and bottom. You need a radiator with passenger side bottom tube - slant or early SB (~1970-). The top tube location is best in the center, but could be on the driver's side if you don't have an AC compressor.