I'm sure Carson at echo archery provides excellent material, but you might wanna consider just buying an Ipe 2x4. A 10 ft piece runs maybe $50 but it's quite a few bows. Maybe 6 or 7 depending how you build them.
Just playing a little devil's advocate, as getting your own supplies is always going to be cheaper, but some things to consider about getting your own lumber vs buying from a reputable vendor:
1) Do you have the materials available locally?
2) Do you have the equipment (or access to) to rip your own boards?
3) Do you know what to look for in a board (not all boards are created equal for making bows)?
4) Are you prepared to spend a decent amount of time looking for proper bow-making materials? There might be a good piece of ipe, hickory, bamboo, etc, in the big hardware store, but be prepared to look through tons of boards to find the right one.
At the end of the day, if all you're looking for is material to make one bamboo backed ipe bow, I would just order the materials online. Since Echo Archery has been mentioned a few times, if you figure in shipping, you'll be spending about $80ish for the pre-flattened bamboo and an ipe board. While this is more expensive than just buying a good ipe and hickory board on your own (or finding bamboo that will work), you'll not only be getting Grade A materials that will do their job if you do yours, but you're also supporting a business that supports PA (and also is active on this forum as well).
Just my $.02, take it for what it's worth.