Looking at cutting my own topwater lures. Does anyone know where to get balsa wood that is thick enough for most lures?

Try here:

http://www.nationalbalsa.com/balsa_planks_s/211.htm

I've gotten batches from several sources including National Balsa, all seemed OK.  Balsa density can be 6 to 18 lbs/cu ft.  The least dense is "competition balsa" which is super light stuff for model airplane builders.  While you can use any density if you reinforce it adequately, experienced builders often prefer using "hard balsa" 12-18 lbs/cu ft density because it is more durable.  You may have to pay more to specify the balsa density you want but in the long run it will pay dividends.  IMO, don't skimp on the raw materials because, compared to the work you put into a bait, the cost is inconsequential.