My scooter takes two 17 AH (amp - hour) 12V batteries. There are two or more types of connections - - a slip-on "Fast-on" connector and a lug with a hole - - which you use a nut and bolt to connect. I measured my battery to make sure that the new one will fit. I found that a 20 AH as well as a 22 AH battery is the same size. So, for a few extra bucks I'm going with the highest capacity batteries.
https://www.amazon.com/gp/product/B00K9Q6LDU/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1
To make sure that you select the correct size battery and with the right terminals, you'll need to open the battery case. Mine took 8 Philips screws to open it up.
Both eBay and Amazon have a bunch. There are also companies that advertise that they sell batteries but they are just a middleman - - like Walmart, Home Depot, etc. You can also buy from a local scooter company and they may install them into the battery box - - but they may charge you $150 or more. For the 18 AH batteries, it seemed like a good price was $40 each. For the 20 AH ones it seemed like $45 was decent. I bought a pair of batteries from Amazon for around $90 or 95 plus I bought a goofy 2 year insurance policy. With tax the total was around $105 (free shipping). I am supposed to get the two batteries delivered tomorrow. Hopefully no drama.
BTW, I've read that for scooter batteries (Seal-lead-acid [SLA], Absorbant Glass Mat [AGM]) that they live the longest lives if you don't discharge them too deeply and keep them charged up. I've read that typical life of scooter batteries is 2-3 years (depending on usage, deep discharges, etc.). There are also deep discharge batteries that may be used on electric boat trolling motors, but I'm not sure the difference. I suspect that they may be larger in size and not direct swaps for the scooter batteries.