Harbor Freight, Sears, Radio Shack come to mind.

If they are "capped off", chances are that the heads will have been removed and unless the system was piped in metal, a metal detector will be of little use, for this application.
You may find caps when the ground cover has been removed, or using a little logic may direct you to a probable location.

Do you want to make these heads functional or do you want to get rid of them?

You are right!! They are capped off so there goes the metal detector idea.

I want the heads to be functional. I'm planning to rake out all the wood chips and seed it eventually. But I wanted to locate the sprinklers first.

the lines should be pretty close to the surface. Have you started removing the wood chips yet? If not, I suspect you might find the sprinkler lines when you do.

These pipe runs are generally straight in an open area. If you can find the heads before and after the buried units, you should be able to at least get a line to search. The heads are generally placed so you get head to head coverage. Measure 1/3 the distance between the heads and carefully dig down. If this doesn't work, dig a trench from the head you see until you get to the plugged riser. Add a head and trench along to the next one.
Sometimes it's not easy.
Ron