Like everyone else has typed Craig's List is your friend.
http://craigslist.org
Pick your state and the closest city and do searches for all of the following individually. aquarium, aquariums, fish tank, fish tanks, freshwater, fresh water, saltwater, salt water. Do the searches in the "for sale" section and the "community" section.
You should be able to find a 75 gallon tank by itself or complete. You may even be able to find a bigger tank for cheap. A 90 gallon tank is the same footprint as a 75 just 24-25" tall instead of 20" tall.
Earlier today in my area there was someone selling an "80 gallon" with stand and canopy for $65. I didn't have the money or I would have emailed them. I've got too many tanks anyway. Lol.