I'm currently running tubes at least until the funding comes in to switch to tubeless, but I need to get a spare to carry with on longer rides. I know 27.5 tubes exist, but options look pretty limited. Most people I talk to are stretching 26" tubes to fit or using 29er tubes. Any thoughts on the Pros/Cons of this?
This seems to be a non-issue...If you read what people say in the formus about using the wrong size tubes. Doesn't appear to make any difference. But I'd still buy the correct size if I had the choice. As for carrying a spare tube I doubt a 26" tube will weigh much less but will be a few grams lighter. The Maxxis welterweight tubes are regarded as being good tubes, and cheap at least over this side of the pond, I bought some 27.5 ones for £3.70 each with free postage from France to UK.
If you really can't find 27.5 tubes that you like, and you have to choose between a 26 or a 29 tube, i'd go for thick 26 tubes (like the cheapest kind you can buy). These should stretch into a 27.5 rim and provided they are thick enough, will work fine.
On the other hand, i'd imagine a 29 tube would overlap inside your tire when inflated - which could be a source of friction that would easily create a tear or rupture.
And, for a little perspective on tubes and dumb things you can do with them if you know how to do it (which isn't hard) and you're in a pinch: When I was still working in a shop some years back, I had a customer burn through what few 14" tubes we had trying to change their kid's flat on their own, so I offered to just fix it for free if they bought another tube because I didn't want them knowing I was going to throw a 10" in there (because people are a lot pickier than they needed to be for some junk kid bike, and we were out of 12's for some reason). Worked fine and they didn't end up coming back in for that bike again. The only time I've ever seen any sort of issues with stepping up sizes was when someone was trying to use a somewhat narrower tube into a significantly wider, larger diameter tire--and stepping down a size with the aforementioned overlap issues.