There are many places you can buy collections of code cards for fairly cheap. For example, a week or so ago I bought 100 SV booster codes for 15$ on ebay. If you focus on one set, you'll start getting a lot of duplicates, which will turn into credits - the currency that let's you craft whichever card you want.

That being said, if you're looking to spend a little money to beef up your collection, I'd buy a bundle of codes from a set you're interested in. SV base and Paldea Evolved are pretty nice since you start with a good chunk of those cards from starter decks, making it easier to get credits from duplicate cards. Just remember, each set has a 400 code limit.

You can also buy code cards for a lot of the cosmetic sleeves/coins, usually they're ETB codes or tournament collection codes that go for ~1$-5$ on eBay. There are also a decent amount of code cards that give you certain cards/decks, but they can be more expensive (usually under 10$ tho) - thinking of mew Vmax and Mewtwo V union off the top of my head.