Most sports shops these days sell them, I'd advise going for a softer foam to begin with if you havent done it before. You can also use things like tennis balls or just massaging with your hands is going to help improve the situation if you are not doing any of that kind of work.

Probe around with your thumbs or put pressure on different muscles and try to find the places where it is hurting and the muscles feel twisted and that is the area you want to concentrate on.

For me with knee pain it is where my quad comes in to meet the knee on the inside of the leg that causes me all of my issues but that won't necessarily be the same for you.