are good places to start for range fed meat.
About mice. They are opportunistic omnivores, much like us. In thier natural environment grubs, seeds, carrion make up the diet (again, much like us!). As humans developed graineries mice and other critters capable of eating this dense easily accesable food source did so. The association of mice and humans with grain is purely a result of the agricultural revolution and is certianly not a reflection of the "natural" world.
The point you raise about prions is intriguing on so many levels and really illustrates the no-mans land that a paleo diet exists in. On one side we have american big business which will subject the natural world and consequently us to anything so long as a profit is obtained (ruminants were never designed to eat grains OR meat byproducts...both are foreign and have the potential for disease).
on the other side we have the "enlightened" vegetarian movement which has been pushing an agenda which might stave off ecological collapse, but is killing us in the process.
Really no one wants to hear that we function best on lean, wild protein sources of animal origin, vegetables, fruit, nuts and seeds (all of these local and in season---no watermellon in december for north america!). These are the most perishible, lowest profit margin items around. All of the SnackWells, luna bars, canned and boxed goods are the profit generators.
BTW-making a diet based off poultry and fish and avoiding mammal meat is a perfectly viable thing to do.