As an ASE Master mechanic for the best part of 30 years I want to give you a little advice about todays auto parts stores. Shadow stated " just go to an auto parts store and ask a clerk to pull a few of them for you " and he also said "You can also have him show you transmission fluid coolers.......they come in a few different sizes" .
It is a shame that in todays auto parts market place service like that suggested above is almost impossible to get. The exception would be if you deal with several suppliers and spend a few thousand bucks a month with each of them. Even after this I sometimes get the feeling I am bothering the guy if I ask them to send something over that is not for a specific application.
I hope you get someone open minded enough to understand that a heater core can be used for many things.
With all that off my chest, before I purchased a cooling cube from Danger Den, I would have used the heater core for a Chevy chevette. It is cheap and just about the right size.