Hey there! I'm actually pretty new to using driftwood myself, but I've been doing some research and just bought my first piece last night. I got mine from eBay, a piece of malaysian driftwood, whose dimensions are 10x7x6in, for $25. The shipping was free. We'll see how it is once I get it in the mail. They have some smaller pieces that are around $20. The seller was betta-factory. I have a 5.5 gal too.
I've snooped around and it seems that malaysian, mopani, and manzanita are pretty good types of wood to use, but I have no experience myself, sorry.