Manure

It's actually herbicide rather than pesticide residues that could be a problem.

You need to find locations in your area which might be happy to give away their manure, but whether it be horse manure from stables, farm manure or manure from other animals, I would strongly recommend talking to the owner if possible to check whether herbicides were used on the grazing land or in other feed materials.

Good advice from JayG, when you collect take a look also if you can at the pasture where the animals are kept, there should be plenty of weeds. Herbicide can come in however on winter feed without the owners knowledge, unfortunately while ever this weedkiller's used it's going to be a game of poo roulette for us gardeners.