Garage sales and used book stores. Ebay sometimes, christianbook.com, amazon.com, alibris.com I'm always looking for used Bibles.
I don't meant to brag but for some reason Bibles find their way to me on their own and usually inexpensive or free. A few have cost some coin though. I have several study Bibles that are collectable and worth way more than I paid for them. Once I won a New King James Study Bible on ebay. It was supposed to be a hard cover, I won it for $6.00 When I got it, it turned out to be genuine leather. I was given a Dickson Btudy Bible, this is worth $400.00 in the condition it is in.
I have also gotten burned a few times. Then again, I have a 1st edition kingdom interlinier that i paid $3.00 at a used book store. On the topic of interliniers, a previous poster mentioned one which is good, another good one is "The Reverse Interlinier" which is based on the NIV. A strongs concordance is also a must because it assigns numbers to the greek and hebrew words so that you can look up the word in a greek or hebrew dictionary or lexicon.