I have dealt with Jack First, and agree, but he has nowhere the inventory of parts for old and obsolete guns that Numrich has. I wish I had a dollar for every time I heard First and others tell me they never heard of some gun that Numrich had parts for in their catalog. Yes, I have had to deal with bad repro parts (IMHO, Dixie is far worse than Numrich in that respect), and spent too much time fitting Wolff springs, but it still beats making the part. Other small suppliers are specialized, and often have a poor selection. Some never have what is wanted, others charge high prices for broken or worn out parts and won't even discuss refunds.
On the other hand, I have lucked out with Numrich. I once ordered a firing pin for a M1903 hammer Colt and got an original, still in the century-old waxed cloth wrapping! In general, I have had satisfaction with them. As a gunsmith when I dealt with current guns, or guns "with a home", though, I always preferred to order from the factory if possible. I was more likely to get the right part and to talk to people who knew what I needed.
Some gunsmiths (and one of my bosses) would never order parts until he had enough to get some discount or other. That was OK except that the customer fumed for weeks because his gun wasn't fixed, and the boss would claim the parts had been ordered (a lie) and had "not come in." When I could, I preferred to just order the part; the customer didn't really care if it cost $5 or $7 if his gun was fixed. But the penny-pinching boss retired rich, so I guess he knew what he was doing.
Jim