Ebay and the jyard are indeed the best place...
Best in OEM to look for... hmm...there are a number of good ones. The SAAB blackstone all aluminum version (there is a plastic tank version) is good, Starion intercooler is nice, Ford Probe GT unit...not bad...
Also, any number of intercoolers out there with plastic tanks can have those tanks removed and aluminum ones fabbed welded on... if your handy with the fabbing and welding skills there.
Also, spearco cores and other cores are available all the time for VERY decent prices on ebay and are very nice flowing cores...the endtanks for those have to be fabbed and welded on as well.
I've seen several guys take and run dual G60 intercoolers with custom endtanks (but fit OEM bracket mounts) on their setups (gas and diesel) as well as TDI cores, audi 5k cores (there was a guy on the net making stock 5k intercoolers into aluminum end tank units and reselling...don't know if he is still around).
Lots of options....time and money are the question
Corky Bell's book "Maximum Boost" has some do's and don't' s for intercooler end tank flow/design as well as core construction for max flow and efficiency if you want to do some reading
Joe