You have to be "creative" about the whole process and REALLY think it over.
For standard size tanks, there's just no way you can build one for anything approaching what seems like a ridiculous LFS price for a tank. So if money is the motivator and you don't have any oddball size requirements, buy used or wait for the Petco sale.
If you have some oddball size requirement that makes store-bought tanks just out of the question and money is still an issue, you need to scour craigslist, newspapers and garage sales for used and broken tanks. Yes, if you cruise the better neighborhoods when they have bulk trash pickup days, you may find something there too. Dismantling any size tank isn't for the faint of heart and takes a LOT of work. So what you save in material, you will give back two-fold in scrounging material that you have to make usable.
I think in the end you'll find Craigslist, classifieds, and used tanks will be the most useful tools for saving a buck.
I forgot to add that a measure of good luck is always helpful too.
"Why can't my wife see all of this stuff as an investment?"