Typically, you need to place your order well in advance and a deposit may be required. If you are already using a farmer for pastured chickens, check to see if they will have turkeys. Another option is to check with your CSA. I know my CSA offers turkeys to members on a lottery system. We order our turkeys from Farmer Nick at the Crystal Lake Farmer's market. He'll take a $10 deposit and let you pick the size range of your bird. Find other local sources here.
If you'd like a little overview on what exactly a hertiage turkey is, check out the Heritage Turkey Foundation's website.
Word of caution, pastured poultry cooks faster than your average factory farmed bird. This is true for chicken as well. And I mean A LOT faster. They are typically leaner and smaller than the grocery birds. No need to cover the breast with foil while roasting. You should cook your heritage breed turkey at 425-450 degrees until the internal temp (taken at the fatest part of the thigh, making sure your thermometer is NOT touching any bone) reaches 140 degrees and then take it out of the oven, cover and let it rest for 10 minutes. Also, if you plan to stuff the bird you will need to precook your stuffing because given the reduced cooking time, your stuffing will NOT be fully cooked if you don't precook it.
Here is a specific recipe you can try when cooking your heritage breed turkey, but it doesn't have to be complicated. I just cover in butter, add a little salt and pepper, and put in my roasting pan. No brine or anything. They always come out juicy.