I've been looking for a Canon 17-55mm 2.8, possibly the best EF-S lens made by Canon.

I usually find my gear at a price on par with BHphotovideo store in America, but for some reason thai shops are charging a bit more for this particular lens, and without giving the Canon warranty.

Here (by the time of this post) it can be found for 879 dollars, which converts to around 27250 baht (as of today) http://www.bhphotovideo.com/c/product/425812-USA/Canon_1242B002AA_EF_S_17_55mm_f_2_8_IS.html

I've looked 3 online catalogs from shops considered cheap at Fortune Town, they usually have the best prices (but often don't give you the original warranty paper), and even at these shops I can't get it for less than 29900 baht (official Canon shop is way over that, around 36000 baht if I remember correctly)

Anywhere else I should be looking?

You could try ground floor at MBK (Fotofile). They sell good used glasses, with warranty too. Must bargain a bit.

FotoFile lists it at 34,500, on sale.

I bought the Sigma 17-70 2.8~4 macro for under 15,000 baht (about the same as BH Photo) and have been quite pleased with it. Sure, you lose a stop at 50 mm. But then, you gain macro capability and 15 extra mm focal length!