Didnt look to see if there was anything available but you can do a search for aquarium in the "For Sale" catagory here-->http://portland.craigslist.org/
. (and be sure to browse through the "Free" section!)
Maybe look on Ebay. Narrow your search on the left to limit the distance from your area code.
Other than that, it would depend greatly on how nice you want it to look. If you want it to look like a piece of furniture, well, you're going to have to shell out extra. Either to a LFS
or a local cabinet maker. If you want it to function and are not too worried about the looks, it would be cheaper to make one yourself (not saying you have no skills, just basing this on the majority of people). Or, you could possibly have a local handyman build one.
Just my suggestions. I am sure there will be others.
If you have the tools and a bit of knowledge, you could probably build one for less than $75. More if you chose to use cabinet grade materials.