Those tyre repair kits are very common in South Africa. All the outdoor shops sell them. I bought one last time I was over there, and that's my post-SHTF solution, along with a compressor.
A compressor is a good item to have, as it allows you to drop your tyre pressures for dealing with difficult terrain. Even a 2WD car can cope with slippery conditions if you drop the tyre pressures to increase flotation. When buying a compressor, don't be tempted to buy ultra-cheap. Many cheap compressors are sold on the basis of delivering a good pressure (psi), but for tyre inflation, volume of air delivered per second is also important. A compressor that's not up to the job will overheat during the extended time it takes to inflate just one tyre. (I know from experience!)
Post-SHTF, serviceable pneumatic tyres will still be useful IMO, even without road fuel. A trailer equipped with pneumatic tyres will make better headway over muddy, rutted ground compared to wooden wheels, even if pulled by a horse (or team of people, or dogs)!
Re. tubes for Landy wheel sizes, yes you can get them:
http://www.allterraintyres.co.uk/index.p..._1263_1296
Find a resilient place and way to live, then sit back and watch a momentous period in history unfold.