Your choices, which are not many, will depend on the type of triggerfish you want to keep. Triggers with pointed faces will eat almost anything you put in the tank as a clean up crew so YOU will have to be that crew. Roundfaced triggers will also eat almost anything but you may have a chance with Turbo snails or other snails. Crabs, Urchins, Shrimps Cucumbers, etc are all food for any triggerfish. The big issue with using snails is that if they are killed and not eaten, they can pollute the water.
When dealing with triggerfish, you are best being the clean up crew. Make sure you clean around rocks and crevices as they will fill up with detritus and pollute the tank.
Hope this helps.