Alright, let's get into things...
Where does one buy a pig?
Whole hogs can be specialty ordered from the farm itself where the animal has lived, a good neighborhood butcher, or at the meat counter of your local quality supermarket or co-op. Ask around first at the butcher and work your way onwards from there. If they can't get you a whole pig, chances are they know who can, or point you to the pig farm where they buy their meat from.
I have paid as much as $4.00/lb and as little as $2.50/lb for a whole pig. Organic pigs can sell for significantly more depending on the source. You pretty much get what you pay for when you buy a pig, so do a little leg work and chose wisely when ordering.
Some seasoned pig roasters will recommend about 1 lb. of hanging weight pig per person attending the party. I have found that ratio to be much too low. The math on that estimate yields around 6 oz. of cooked pork per person. I'm not going to say publicly how many ounces of meat I expect to eat when I go to a pig roast, but I'll tell you it's certainly more than six. I recommend doubling this sizing guideline and figuring on 2 lbs of hanging weight pig per person attending the party...roasted pork makes great leftovers and soups, so if you go a little overboard, there's no reason anything needs to be wasted.
Fresh or Frozen
The pig may come frozen, but hopefully it will be fresh. If it's been frozen then it will need to defrost over 24 hours or so. DO NOT ROAST A FROZEN PIG. Place it in a safe place where animals can't get to it, wrapped in plastic, and let it thaw. A big plastic tub works well as a holding vessle, or the bathtub, or in a cardboard box in the garage located such that if some juices come out as it thaws that it won't make a mess. If you your pig will come frozen, order it for the day before your pig roast so you can defrost it.
If it's fresh and not frozen, that's great! Simply keep the pig refrigerated, in a cool place, or on ice in a cooler until the morning of the pig roast. Since it's so large, I have learned that having a spare fridge on hand can be nice. Remove the racks from the fridge, place the pig inside, and shut the door. If your pig will come fresh, order it to be picked up on the morning of your pig roast and then you won't have to deal with the "where do I store a whole pig" dilemma.
Regardless of whether you defrosted your pig or not, remove it from the fridge/cooler an hour or so before you are ready to place it on the spit since it's not proper form to cook cold meat.
Roasting pigs are young pigs - usually between 30 and 60 pounds, however they can come larger. Pigs that are used to make bacon are generally hundreds of pounds, however that's not a great roasting pig since the meat is older and tougher, so stay away from anything that's over 100 pounds if you're looking for tender juicy meat - additionally, at that size, it just becomes unmanageable. Better to get a second or third smaller pig for your roast.
A 50 pound pig cooks in anywhere from 4 to 7 or even 8 hours depending on your heat source and whether or not you've stuffed it with anything...more on that later. Some fellow pig roasters recommend around 1 hour and 15 minutes per 10 pounds of dead weight pig. I have found that it's actually pretty variable depending on the heat from the fire, the height of the spit above the flames, if the pig is stuffed, and if you are using a motor driven rotary spit, or rotating by hand.
In general, work backwards from when you'd like to eat using the 1hr 15m guideline per 10 pounds of hanging weight pig and add in an hour or so for carving and all the things that take longer than you've planned just to be safe.
Sources for Pigs in the San Francisco Bay Area
Just recently I purchased a whole pig from Ver Brugge Meat, Fish and Poultry
in Oakland, CA. Last year I we purchased a tasty pig from The UC Davis Meat Lab
in Davis, CA. Whole Foods, and other grocery stores in the area can often special order whole pigs as well. As I said before however, going direct to the pig farm is best and you'll likely avoid the butcher's mark up. If you are in the bay area there are several local pig farms to choose from. Although, this article makes a compelling argument
as to why it's better to buy a midwest pig as opposed to a local one. Long story short there is that it takes less carbon to feed the pig local grain in the midwest and ship the dead animal to California than to ship 4 times as much midwestern grain to pigs out west