I'm really surprised they're hard or impossible to find where you are. Cassoulet isn't usually vegetarian (although it can be), but it's a traditional French dish with many variations, and I would have thought pintos would be used sometimes.
Would it be possible for you to grow your own pintos? I've been trying various types of beans out here in upstate New York, and so far, I seem to have the best luck with black beans and blackeyed peas- the blackeyed peas don't produce that much, but all the critters that usually savage my other beans are evidently afraid of blackeyes. I think I've had some pintos grow, too- but unfortunately, garbanzos don't do well here.
Beans aren't popular overhere and cassoulet is a regional dish from Castelaundry (South west) made with
haricots lingot - they are huge white beans. Most people gorge on meat, dairy and delictessen meats overhere.
I have asked every single shop in town and area (including Organic and oriental stores) and people don't
even know what pinto beans are.
I don't have a garden and would imagine you'd have to have a huge one to grow beans.
Anyway, I have given up and stock up when I'm in the UK.