SF Nematodes on amazon → http://amzn.to/2iKZiZo
Hb & Sc Nematodes → http://amzn.to/2iL2xQx
A different brand with good reviews → http://amzn.to/2io2w4j
When you apply them is very important. You will get better results if you do it early in the year after the last frost. The winter cold will kill many nematodes close to the surface but the ones hiding deeper will survive and multiply.