Well, it's the USDA that won't allow certain parts to be used for food rather than the FDA. That said, pig stomach is a good substitute and if you can't find it at your local market, try a Mexican market. Pig stomachs are easy to find here in TX. I use the pig stomach when I make venison haggis as it's easier to work with than the deer's stomach. I've got several pounds of haggis in the freezer, venison and wild hog, and it gets served for Sunday breakfast every 4-6 weeks with porrige.

Never had vegetarian haggis. I think I'll stick with what I have.

YMOS,
Tony