One option is to find a friendly auto parts store-
They may let you look at various heater cores until you find one that suits; I got my Chrysler core that way.
Things to look for:
copper/brass construction
flat-tube (as opposed to round copper tube with pressed fins)
barb size (a good core will have barbs sized just right to fit the tube you use tightly with NO MODS.)
Single pass is better than dual pass.
To explain the last a bit-
water going down one side of the core and back up the other side is a dual-pass and less effective.
The best cores/radiators for our purposes have the coolant going through the unit in a single pass: in one side and out the other.
Not to say that a dual-pass won't work of course. My big Crysler unit is one and does just fine, but if it was a single pass unit my temps would be slightly lower.
Perhaps even enough to actually see a difference.
