ffutures | Natural vegetable crisps

Question for my UK veggy / vegan friends (congrats on the Mars thing, incidentally), and for anyone else interested.

We have a vending machine at work that sells "Snapz" crisps, which are basically thinly sliced dried vegetables. One of the varieties is beetroot, and I've just tried extracting the natural colour and using it as an acid/alkali indicator, as you can with normal beetroot. It turns out to work really well, and is much easier than the original experiment. The technical details? Add boiling water, soak for a minute or two, and pour off the liqud. You now have a dye that's red with acid, yellow with alkali, and doesn't stink of pickled beetroot or red cabbage (which is the other one we sometimes use).

That's all well and good, but what I really want to do is extend it by adding more varieties of vegetable crisp and get more indicators, so that we can do some sort of class investigation. I've tried carrot and it doesn't work - the dye isn't an indicator. And the vending machine only has those two and apple with various flavourings.

So what I'm looking for is some other sources for natural crisps - can't believe that carrot and beetroot are the only ones available. Any suggestions? Needs to be sold in the UK, of course, and if possible reasonably cheap. What I don't want is potato with flavourings, apple with flavourings, etc.

Thanks!