Go to your garden and get them. You should have several species, and grab some earthworms while you're at it. If your garden is relatively protected, urban, no wildlife (herp's, raccoons etc) then parasite burden is probably minimal at most. This is even moreso if the surrounding area is very urbanized with little wildlife (so if the odd raccoon enters, it hasn't traveled in a herp-orientated parasitic burden area). If you are still concerned about parasitism, or you are from a more rural area, still grab your garden critters and captive breed them. The parasites that they have, being intermediate hosts, will not be vertically transmitted (parent to offspring), or horizontally transmitted (bug to bug) since the breeding stage of the parasite will be in the reptile or above (the reptile may also be an intermediate host). And even around all this, have regular fecal's done on your snakes, as they are more likely to get parasites from the food that you are feeding (if not frozen/thawed only).

Ian