Amazon is really not the place to shop for these, but here are a couple of decent options. They're all pretty cheaply made, though
Interlock 3 - not too hard and not too expensive - really fun and might be a good starting place
Burr Box but also available from Creative Crafthouse, I think
Searching Amazon for Creative Crafthouse, you'll also see they carry a couple of 18-piece burrs, but those aren't the place to start.
I highly recommend you look at Puzzle Master unless you're not in North america, but then there will be other options