Seems there are always 40s fairly inexpensive. In the last year, I bought a upper and lower from Outside Outfitters and the same from Jenson. I think I price matched Jenson using Outside, you can get free shipping from Jenson. And there are usually good deals for both 40 and 110 coming up on ebay occasionally.
If you do not already know, you will likely need to buy the upper and lower assemblies separately.