Search on
TF2 Outpost or
Steam Trades for listings. There's also
Dispenser.tf, but not everybody chooses to use trade bots so the former two are the larger resources. With TF2 Outpost, the game you want goes in the left-hand section while what you want to use to pay for it goes in the right (you can select "Real money" if you'd prefer PayPal, though for the most part people just mention the PayPal equivalent in the listing description despite having only keys listed as the preferred payment method). If you'd rather not pay market prices, there is no shortage of second-hand keys on TF2 Outpost that you can pick up for <$2 each, however as I've said before it can be a bit of a pain to find someone who's actually online and doesn't enforce rules pertaining to trade rep or amount of time spent in TF2.
Just remember to exercise some common sense: listings created using accounts that were made recently or older ones with few trades are ones you'd do well to steer clear from, and remember that each TF2 key is essentially worth $2, so if you see, say, a listing for The Witcher 3 and the person wants only 10 keys, it's obviously a scam as TW3, at its cheapest, converts to approximately $24.40. You can use
SteamDB to see the price of a game in different regions, so to continue with the aforementioned example,
TW3 has a base price of 1049 Russiabucks and there's a 20% discount given to those who own both previous Witcher games, which brings it down to 839.2 Russiabucks (~$24.40 as mentioned earlier). Once you've got the converted price you basically want to round up to the nearest $2, hence the price of 13 keys for TW3, though it probably goes without saying that if a price converts close to the nearest $2 mark then you'll have to round up further still (e.g. you'll be looking at 14 keys for something that converts to, say, $25.90). Some traders accept
ref in addition to keys (if keys are dollars then think of ref as cents), but there's not much need to dive into that.
The good thing about "buying" games with keys rather than PayPal is that if something goes awry you can ask Valve to reverse the trade, whereas with PayPal you'd have to bark up a very long tree.
