If i is something I really want, and it is not on bookmooch, and I have the extra cash, I will buy it from amazon or ebay. Occasionally I wll order a book directly from the author. that is how I purchased Creature from jekyll Island and Badnarik's Good to be King, both were autographed
I usually look for books on Amazon, and then head over to used.addall.com which is a metasearch engine of used book networks. It also kind of depends on what it is and quite often Amazon's free shipping and minimal price difference gets me to buy there. Also you have to be careful about what edition you're getting if there are more than one out.
Also tend to order a fair amount from Amazon.co.uk and from obscure overseas publishers and obscure domestic publishers that Amazon doesn't carry.
Used to love Loompanics, but they went out of business.