Loras isn't just a knight, hes a Lord, and heir to the incomes of Highgarden. Thats a lot of money. For now, his father and grandmother probably take on his expenses.
Service without pay is indentured servitude (slavery-lite, thralldom). or outright slavery. Dany probably doesn't approve of these. Jaime would be paid as a kingsguard and also have access to Lannister finances like Tyrion. Barriston and Jorah would be paid as well. In times of desperation they may forego pay in an act of loyalty, love, or in expectation of full payment and favor afterwards. Or to keep spying.
Brienne would be paid by whoever she is currently working for. But they keep dying. Also, she keeps getting sent on long trips, so she'll be given a lump sum beforehand. The reason she hunts and sleeps outside is because, particularly in post war Westeros, there simply isn't a steady access to room and board at all points on the road, and hunting an animal can get you a decent meal for free (never know when that next purse of gold is coming, or if your employer will still be alive), and at times, sleeping outside may be safer (and possibly cleaner...) than sleeping in an inn, depending on the inn.
Squires like Pod are like interns. But he does get room and board provided by either the Crown or House Lannister. Whoever arranged for Pod to be Tyrion's squire (Tywin, it seems) would have made the arrangement.
Selwyn Tarth would certainly provide Brienne if she was truly in need, but given her relationship with Jaime she would have room and board provided by the crown. And a kick ass new set of armor. And a Pod.
The Hound became an outlaw for defection. He had some money left over for being paid in his service to House Lannister but it was running thin by the time he fell into the Brotherhood's hands. But yeah, hes already wanted dead or alive, so fuck it, steal what he needs until he can find something to do. He was trying to ransom Arya, after all, and he might have found service in another house. Most knights, however, do not steal from peasants - those peasants pay taxes and rents to someone, and that someone would not be happy with getting leeched off by a wandering knight.
Knights are either landed (they own land, they charge rent for people to work the land, they get income), employed (guards for a Lord, position of office, master-at-arms for a castle, city watch commander, etc) or in active service (wartime).
Knights were paid well for war, they were (in much of European medieval warfare) the backbone of the army. However, they were acutely aware of how much money there was to be made for their service in war not from their Lord. Knights were really the only ones equipped to take down another knight or lord, and if they were lucky, they would get them alive, and afford them the opportunity to ransom. ofc if its the King or something your king is going to (if hes nice) thank you, pay you and take the King into his custody.
King John II of France was ransomed for 3 million ecus and half of France itself - ransom is serious business and hugely profitable if you get the right person. If you were a bit of a dick you could also strip your captured knight or noble of their gear (worth years of wages) and horse and sell them (this happened in tournaments melees too).