Where can I find criticisms of John T. Koch's assertion that TARTESSIAN is a Celtic language?

First of all, if you haven't already seen this article, I recomment it as a starting point: http://en.wikipedia.org/wiki/Tartessian_language The section on "Classification" and then the referecnes at the end may be helpful.. Evidently one or two others had the Celtic idea before Koch, and then since the publication of the book he has co-authored two articles onthe same subject. You may find in them mention of scholarly criticism to which he's responding.

Then, have you tried the MLA Bibliography? Although Tartessian certainly isn't a modern language, there just may be something about Koch's work listed. Otherwise, I think your best bet is to go to a big university library that subscribes to many scholarly journals and start combing those in the linguistics field.

There may even be a journal or two devoted to Celtic studies! Someone likely to know is Dr. Joseph P. Clancy, a professor emeritus at Marymount Manhattan College. As of about two years ago, he was living in Wales, as this article tells: http://www.highbeam.com/doc/1G1-83757456.html I even found an address for him there, but I'd rather not post it here. If you'll email me, I'll give it to you.

Good luck!

BTW, has anyone suggested that Tartessian was related to Basque?

Have just glanced at Koch's article in Palaeohispanica 9, in order to get your drift... A good start might be to search out, not opposition to Koch [since that article was just published last year, it'll take a little time for learned journals to publish opposition] but to have a look at material opposing the notion of a Celt-Iberian language separate from the Celtic and Italic branches of IE. Which Koch presumably believes in.

You need access to a good university library - one with a Celtic and a Historical linguistics department.