They have been available in the past on eBay for cheap (like ~$2 to $3 each) but you generally have to replace the battery as these devices were manufactured quite a while ago and they batteries in them have failed/won’t take or hold a charge. The eBay seller a lot of us got them from is out of stock.
Battery replacement is very simple and only costs a few bucks. I did it on five Minimote V1s I purchased on eBay (the batteries are available on eBay as well).
Minimote battery replacement info:
Unfortunately, the vendor on eBay who used to sell Minimotes v1 is out of stock:
Cheapest I see on eBay is $15 each in a two-pack, but remember you’ll want to also buy the batteries on eBay as well if you buy the Minimotes, as it’s highly likely the batteries are DOA on these.