Also available for about $5 from China via ebay if you can wait six weeks or more. If your e-bike has an LCD display, they're not needed.
They do have a limited transmit distance. I've had to move the sensor as high as possible on the wheel to get the display to read reliably. . On my folder with a 20" wheel, it just seems to be enough with the sensor mounted on the spoke nipple.
Last year I bought two of these Sigma BC 5.12 computers on EBay for about $18 each. It is easy to install and easy to read while riding. It shows just about everything you would want to see, and can be removed from the handlebars with just a twist. There is no backlight, so if that is something you need, check out the BC 16.12.
This is the 5.12 model. Sigma has a very good website.