It’s not a project for most people, but watching someone ride up to 20 miles, and up to 30 mph, on tossed-out vape batteries is transfixing.
Disposable vapes are indefensible. Many, or maybe most, of them contain rechargeable lithium-ion batteries, but manufacturers prefer to sell new ones.
To make a point about how wasteful this practice is—and to also make a pretty rad project and video—Chris Doel took 130 disposable vape batteries (the bigger "3,500 puff" types with model 20400 cells) found littered at a music festival and converted them into a 48-volt, 1,500-watt e-bike battery, one that powered an e-bike with almost no pedaling more than 20 miles. You can see the whole build and watch Doel zoom along trails on his YouTube video.
You pretty much should only buy one from a shop that has a physical location near you and can do repairs. Like everybody around me sells Trek, so if I ever got one, it'd be a Trek with a Bosch motor. Bike shops will not repair ebikes they don't sell, even though they'll repair regular bikes. And neither Trek nor Bosch are going anywhere.
I wouldn't worry about it too much. The bikes are very simple and the battery pack can be rebuilt. Any decent bike shop should be able to repair an orphaned e-bike.
Perhaps where you live. I recently helped a young man get an e-bike, (somewhat mentally handicapped-- we raised funds to purchase the bike to get him to have better mobility), We got him an Aventon Cargo bike. The local bike shop plainly stated they would work on any e-bike you brought in. And that that all of the area bike shops were the same.
Engwe has been around for some time, have one of their bikes (M20). If a part fails, you just need to send them an email, and they'll give you a price and picture of the part and they'll send it out to you. I have had to replace my rear fender and the front light once and they've arrived quite fast (to Europe). Although they aren't high quality bikes, they're priced accordingly.
I'm going to build one. Total cost will be >$1k for the mid drive motor and battery (not counting the bike itself), but it'll be way faster than the street legal stuff they sell at stores. They're dead simple since they're just bikes with a motor and battery pack, so any shop could work on it.
If you want something cheaper, there are other simple retrofit options as well.
Proprietary spare parts. The motors might be off the shelf so you could grab parts from a different manufacturer. But controllers and batteries are usually proprietary making repairs much more complicated and cost-inefficient