Start off with small places, private stables, livery yards and such. Ask for ferreting first and do a good job, be tidy, be respectful and when the time is right (only you will know) introduce the idea of a dog for marking. Once they are used to that then its a small step to ask to try a bit of lamping, "to mop up a few that you missed ferreting". Once you have the first place the next is easier!
Good luck