Where does one even buy a whole pig for roasting?

city guy.
From a pig farm of course.
Any butcher should be able to hook you up (even in the city), but a lot of farms have their own butchering facilities.
I used to help out with a biker party that we got a pig for each year.
We'd just call the guy and tell him what size we needed.
It's a lot cheaper than you'd expect since all they have to do is gut it & chop the feet off.
From their shop to the back of a truck, to a pig cooker.
Stay up all night drinking, maintaining the fire, and putting out flare-ups.
The next morning you've got tender pork falling off the bone.