DON'T try craigslist in ny for tanks... people here in ny seem to think their used tanks are rimmed with gold. I swear, I've offered people 30 bucks for their 5 year old 40 gallon and they argued with me that it was worth 200. a crazy bunch.
I'd check out the petco at union square, they have a lot of tanks... and Pacific Aquarium on Delancy also has a ton of tanks and seemingly good prices.
is there a reason you want acrylic?
if you must have acrylic, Canal Plastics fabricates pretty much anything in acrylic to your specs. don't know what they'd charge, but you could probably get it drilled etc.
in brooklyn, the petland discounts on 5th ave has cheap tanks... but I've only seen the typical black rimmed glass tanks.
the place in williamsburg, I think it was called Aquatic Creations, claimed they made custom tanks, but I can't figure out if they exist any more, the phone is off... I just haven't been up to N10th street in a while. (not long enough really)
one other thought it to check with some of the food service specialty outlets on bowery... I've seen tanks being loaded in and out of them a few times, probably for lobster tanks for chinese restaurants.