I found this website https://www.bmsbattery.com/ that seems to have good prices on kits and batteries. Anyone have any experience with them?

They are a fairly major seller of Chinese ebike parts, a subsidiary of ecitypower.com, acting as the web presence of the company. They sell a variety of hub motors, like Ananada Cute and Souzhou Bafang, as well as Headway battery packs and a wide assortment of ebike gear. Their service is reasonably reliable albeit less then spectacular. Typical purchases arrive in 3-4 weeks after purchase, and shipping costs can be quite high. I've bought several kits, motors, and controllers etc., with no issues. Everything I've bought proved to be reliable, but they do have some problems communicating in English, and communication can be spotty. It is a legitimate enterprise, with Chinese level customer service it seems, though I shouldn't generalize. I've spent several thousand on various parts from them over the last several years, for various projects. The quoted specs on motors and such should be taken with a grain of salt, for example "RPM" of the motors is often incorrect. They support Paypal which is convenient.