My first suggestion would be to purchase a captive bred one. They are being bred so easily that the cost is very reasonable, and a captive bred will be far less likely to have parasites.
The cost of a single vet visit to deal with parasite problems would be greater than the cost of a young captive bred speciman.
You are right...Black Ratsnakes are great pets, as are the other North American Ratsnakes (which includes cornsnakes). I would check the EVENTS section on the main page to see if there are any reptile shows near you. That's where you will be likely to find quality, value and selection. You can order a ratsnake from one of the many dealers listed, but the cost of shipping will be as much or more than the cost of the snake...therefore, the reptile show angle is your best bet. Avoid pet shops altogether. That's where you will get the LEAST value, quality, etc.
If you are determined to catch one in the wild (again, not your best bet), I would start by finding out exactly where your friend found his. At least you know that they are there. It's kinda late in the season, so you may have to wait until spring or even summer, but you can go looking.
Good luck on your search. I have a normal colored black ratsnake and an albino. Even the albinos are inexpensive.
Check out the cornsnakes, too. Not quite as large. Great pets.
Lots of cool colors. Duffy