I went to greenbikekit and got a kit for $500 several years ago. Here is the same thing with a better battery. Its about 15-18mph in the stock configuration (200rpm). The 260rpm version will get you about 20mph. (also look at bmsbattery elifebike)
36V250W GBK-100R rear driving electric bike kit with light weight and high speed for e-bikes-Greenbikekit.com online store for electric bicycle parts-GreenBikeKit.com
Keep in mind that the prices will be near double the listed price once you add in shipping.