Robinson makes a 12" length of pull tube stock and Maddogdan just shortened his without much trouble.If you look I'm sure you can find his thread.That would be the best way to get your perfect "custom" length.
I am not sure if they sell shorter ones, I know that you can buy a tool to re-thread the buffer tubes (midway has them on sale right now). You could cut down the stock tube and re-thread it to screw into the adapter. Maybe others have some options.
Maybe somebody has answered this before, but I don't see the 12" Entry Stock option available from the RA website. Any idea on its availability and/or the price to upgrade from a folding tube stock?
UPDATE** - Heard from Danica and she let me know that you can get the entry simply by calling and requesting, and it is the same price as the regular folder. Once again, excellent customer service and response from RA.