Your LBS will have 29" tubes for sure. That’s what I use. It weighs much less than a 36" tube too. My only failures have been improper installation where a bit of tube gets pinched between bead and rim, then Kalpow!
Some agressive Pre-stretching the tube helps with the installation, as well as a liberal application of baby powder to the tube prior to installation.
Hey Andy. That looks familiar. . . .this thread could turn into a Nimbus blow-out porn thread . . . . . . . I’ll do my best to keep it porney . . . .
This is from Tour De Tucson 109 mile ride in 2008, actually its my riding buddy’s tire blew the day before the ride and the second pic is the sheet-metal hack that I was planning but didn’t have to pop-rivet on (yes, pop-rivets!!!) because we found a new Coker ribbed tire at the last minute.
Truth be told, this tortured Nimbus Nightrider tire was of the first generation and it had multiple blow-out problems on this first generation Nimbus Stealth rim (with stretched 29er tube and 36er tube both). My friend used to arm this bomb before a ride by pumping it up to about 50 psi and then disarm the bomb after the ride by dropping it down to 25psi. This final blow-out was this tire’s . . . .um . . … auto erotic asphyxiaton where he finally lost his grip in the edge of the step stool in his bedroom closet 
The new Coker tire has been blow-out free.
Last porno pic is actually doll porn from a really huge all girl naked party/orgy at Burning man 2009. . .man those skinny babes know how to mix up some fun when Ken is away. Ye ha!
Brycer
Bryce, are you posting under the influence of controlled substances again ??? 
lolol