I’m a big fan of the Bontrager Latex. Admittedly, I’ve never used any other though. But I’m still a fan!
Edited because I didn’t even answer your question. I think you can order them online but I always just have the LBS special order them. No shipping costs and I have them in a couple days.
What about the super light butyl tubes? Like the Continental Supersonic road tube for example. How would that fair against latex? Although the price is even higher, but is it better? Or have some other advantage?
I haven’t tested the Continental Supersonic butyl tube but comparing the very light Bontrager RXXXL butyl tube to latex shows ~ 5 watts difference on a real road. Butyl doesn’t lose air pressure as fast - if that is a concern (leaving your bike in transition overnight) you’d be better off using the Panaracer Green Lite Poly Urethane tubes which roll almost as fast as latex but hold air pressure almost as well as butyl.