For those who aren't familiar, a SRAM Automatix hub is a 2-sped internal gear hub which shifts automatically, with the shift mechanism and everything contained in the hub. The mechanism is mechanical, and there aren't any batteries or anything involved. So, no cables to the back of the bike needed except for a brake cable if you have a freewheel Automatix (rim brake), disc brake Automatix or roller brake automatix. The coaster brake variant will not need a brake cable, obviously. There are in some sense similar products from Sturmy Archer (Sturmy Archer Kickback, which you backpedal a bit to shift), but the Automatix is the leader of the pack since shifts automatically without any user intervention.
While Automatix hubs are pretty reliable and work well, they aren't very popular in the grand scheme of bicycles. And whats available is highly market dependent.
You're primarily going to find automatix on things marketed as city bikes or something similar. Major manufacturers like Specialized have had them in the past, but you might end up having to go with a more boutique manufacturer (e.g. Mission Cycles). Your bike shop should be able to help you.
The swapping in of an automatix wheel is straightforward but not necessarily trivial; you have to sort out your rear brake situation (coaster, disc or roller in the hub, or have the hub freewheel and use a rim brake), have the right hub spacing for your frame and an appropriate rim size. I'd say its not significantly easier than getting a bike with automatix in the first place.