Aliexpress.com has a lot of TENS units for as little as $5. Only downside is it's over a month for them to get to the states because they are shipping from China. Sometimes stuff doesn't show up, but the dispute process has been solid when that happens. Shipping has always been discreet to date.
Base search for TENS: https://www.aliexpress.com/wholesale?catId=0&initiative_id=SB_20210616120116&SearchText=tens
I got this cheapo unit to start off with which was about $10, 3 years back and is now half the price! https://www.aliexpress.com/item/1005002551120412.html?spm=a2g0o.productlist.0.0.376066dckwwCHz&algo_pvid=281129dc-1362-41f4-8e52-2f018dc678fa&algo_exp_id=281129dc-1362-41f4-8e52-2f018dc678fa-13
It's got a bunch of different sensations and you can switch it to change up the rhythm. Runs on 2 AA batteries.
I originally got it for occasional back muscle relief and quickly wondered what it felt like when used in ways other than intended (but now I'm thinking it was intended for such use)!
The adjustable rubber straps come with the right cable for the above unit: https://www.aliexpress.com/item/1005002599450505.html?spm=a2g0o.productlist.0.0.7273352dMCJQIO&algo_pvid=3fad6ebd-6b8c-427c-bff0-7a40631686e5&algo_exp_id=3fad6ebd-6b8c-427c-bff0-7a40631686e5-13. This gives you up to 4 terminals (2 of each polarity).
I did get a butt plug to try, but never got anything out of it, however, putting a rubber strap on my penis and then using a rubber strap folded in half and inserted in my ass with the ends sticking out of me results in some out of the world sensations.
Thus, $100….more like $15 can get you set up to start some experimentation. I'm sure the more pricey units probably have more features, but this is a great way to get an introduction at a bargain price. I know it got me hooked.
I'm currently (no pun intended) thinking about going to a dual channel system.