This is surprisingly a more difficult answer for people than it should have to be, but the comic market being what it is, means it’s not entirely unexpected.
There’s lots of places to buy comics online. Amazon runs lots of deals and has a very wide selection, but there’s also In Stock Trades, which is entirely comics focused and has even better deals on some titles, and Things From Another World, which includes other merchandise as well, some great discounts, and has the rather appealing ‘Nick and Dent’ section. Want comics on the cheap? Don’t mind getting a slightly used copy? Hit up TFAW’s Nick and Dent for a nice selection of comics that are at LEAST 50 percent off!
Or maybe you want to buy your comics in person, which I support heavily as well - much moreso when the store itself is worth supporting (not all of them are, and I’ll admit that off the bat). You can use the Comic Shop Locator to see if there’s anything close by to you. Some stores run membership discounts, and are usually a great way to pick up tons of back issues - often on the cheap - or just window shop. Check around your own local store, see if it’s a Yea or Nay for you. If you don’t have a local comic shop, usually book stores will have a ‘graphic novel’ section, though whether they angle more towards stuff like Marvel and DC or more towards more indie publishers like Fantagraphics and the like, is entirely up to them.
If you’re interested in buying singles, or following a specific series, you can ask your store to set up a ‘pull list’ for you, where they’ll pull and hold the titles you ask for, for you every month, so you can be guaranteed to have it. Some stores will even give you discounts if you do this! However, make sure you like the way the pull list is handled at your local store. Some handle it well, letting you be as flexible with it as you like, and some can be kind of jerks about cancelling a pull list, or the like.
If you have any further questions, feel free to ask.