The other answers are good, but I would say that vitamins are by far the best way to EV train in SwSh once you can afford them. SwSh removed the restriction of vitamins only working for the first 100 EVs, and will allow you to completely train using them. The two best spots to get them are in Hammerlocke's central Pokemon Center, where you can get them for 2 BP apiece, and in the dojo on the Isle of Armor, where you can get 25 for an extremely cheap 125,000 pokedallars.
The first option is less accessible if you haven't played ranked before, but the BP awards once you get to Ultra or Master Ball Tier (300 and 600 BP, respectively) make it significantly easier, plus you'll get BP just for rising through the ranks. If you've been using Pokemon Home for a while, then you'll probably have a lot of points there that you can convert to BP.
The second option is, naturally, only available if you've bought the DLC, but if you have, then once you've given Honey enough watts (which are easy to get with the watt digger on the Isle), you'll get access to a vending machine that gives you vitamins cheaper than anywhere else in the franchise. There's a good chance you'll be able to afford them just by selling items in your Treasures pocket (once you've started playing ranked, you also get these items every few battles), but if you still can't, then the DLC also gives you access to the Galarian Star Tournament, which takes about 30 minutes and gives you 1,000,000 pokedollars at the end, on top of the prize money you're already getting.