You need to decide if your protagonist and his gunsmith friend are willing to break the law. If there is a revolution going on, civil disorder, etc., and the "authorities" have their hands full already, then a person with a machine shop could probably manufacture a reasonably effective suppressor in a day. For research, buy some of those cheesy "How to Make a Homemade Suppressor" books. The "Chore-Boy" type suppressors are surprisingly effective. You just have to not care that much about the size and weight -- if a suppressor is long enough and fat enough, it will suppress. The professionally manufactured jobs are designed to be as small as possible and still work.
The hardest part in making a homemade suppressor is attaching the can to the gun. First, you need a barrel that sticks out past the barrel bushing, so you can thread it -- and it takes a good machinist to thread it square, so the hole through the suppressor is exactly in line with the path of the bullet - little margin for error. Six-inch barrels are special order items, usually not lying around the shop, so for your story, you might want your protagonist to start with a Commander (4.25") and drop a 5" Government Model barrel in it. Then you need a way to "step down" the rear of the suppressor tube to a diameter small enough to thread on to the barrel. (Again, those threads better be straight.)
If your protagonist goes to a manufacturer of suppressors, he probably has parts or completed suppressors on hand, so there would be no waiting. However, a manufacturer who bypasses the government paperwork risks big trouble -- nobody would ever do it, unless it were an EOTW situation.
I have a Gemtech Blackside on a 1911 (6" threaded barrel). You are right, it is not Hollywood quiet, enabling you to neutralize sentries on a quiet night. To me, it sounds like a solid whack on a snare drum. Not what anybody would call "quiet," but you can stand next to it without wearing hearing protection, and you won't get tinnitus, which you get with an unsuppressed .45. If you were to shoot a suppressor inside a house, people outside the house would probably not hear it. If you were to shoot it in a busy urban area, with traffic, construction, conversation, music, nobody would pay any attention to it, unless they happened to be standing nearby.
The Gemtech Blackside is super-lightweight, so the 1911 will function even with the weight hanging off the end of the barrel. An expedient suppressor would have to be substantially heavier, and the pistol would be a single shot, requiring manual operation of the slide for each shot. Worst would be if it cycled halfway, and you got a feeding, extraction or ejection malfuction. In your story, if the suppressor is homemade, include that factoid -- make your protagonist have to manually cycle for each shot.
Potatoes, soda pop bottles, etc. don't work at all -- except in the movies and in books less well researched than yours.