Try Griffin Radiator
I would give Griffin Radiator a call. I remember them manufacturing cores, although I don't think they do many individual sales. I also believe Modine manufactures intercooler cores, although you'd also probably have to buy in bulk too.
If your looking for intercooler cores, try a wrecking yard for tractor-trailer trucks. Not the best, but their big & cheap. Ask if they have any that are partially wrecked/bent. I recently gave away a huge intercooler from a Hino that was in a small collision. 3/4 of the intercooler was still good, and could easily be cut to make a new one.
...Mike