All really good information above. I'll just add two things. First find the location/park you really want to settle down in prior to plunking down cash. There are so many choices out there to settle for less is a mistake. Go travel around.
Second, IMO it is a buyers market out there. Parents are passing away leaving their winter places for their northern heirs to deal with. New retires are not filling up the parks as fast as the units are coming up for sale.
If you buy something less then what you want with the intent of upgrading in a few years you may find out you're stuck in that same buyers market.
But to answer your OP the best way to find a place is to locate where you want to spend the winters and then call the park office(s). They will normally have a list of units for sale, or know of them.
Thereโs no fool, like an old fool.