With the current shortage of firearms for sale, caused by the coronavirus pandemic and the panic buying it's sparked, I know some people really need a firearm, but can't find one at a price they can afford.
There are two major online sources I've used in the past, that aren't seeking to profiteer from the current panic, and ask fair and reasonable prices for their goods (at least, they do at the time of writing). They are, in alphabetical order:
There are many more online vendors, of course, but I happen to know these two from personal experience, and I've been a repeat customer at both (I bought parts from CDNN just yesterday). I can therefore honestly recommend them, which I can't do for vendors with whom I have no experience. They haven't asked me to promote them, and they're not compensating me in any way for mentioning them.
To buy a gun from them, you'll do an online transaction, then they'll ship the firearm to a licensed dealer near you. There, you'll fill out the necessary paperwork for the required background check, and take delivery when it's been completed.
As for ammunition, both vendors have some available, but that's not their primary stock in trade. It's in short supply almost everywhere right now. Good luck!