The sleeve sizing rabbit hole is real, and most people forget one crucial thing: check what *your specific games* actually use before buying. Standard poker cards (2.5" x 3.5") are common, but I've got games with tiny cards, oversized cards, and weird dimensions all mixed together. If you grab sleeves for poker size and half your collection doesn't fit, you're stuck with a useless stack.
What's worked for me is buying from places that sell in bulk without the ridiculous shipping markup. Amazon's got FFG sleeves and the Dragon Shield/Arcane Tinmen stuff at reasonable prices if you're buying a decent quantity - the per-sleeve cost drops significantly. I also found that Sleeve Kings (sold through various retailers) punch above their weight for the price, and they're sturdy enough that I'm not worried about cards shifting around even after months of play. The big pitfall nobody mentions: don't assume all "standard" sleeves are the same thickness. Some cheap ones are so thin the card still moves, while others are so rigid you can barely shuffle. Hit the middle ground and you're golden.
One more thing - if you're buying for multiple games, resist the urge to get everything at once. Start with your most-played games, figure out what sleeve thickness and tightness actually feels right in your hands, *then* stock up for the rest. Saves you from buying 500 sleeves that feel wrong.