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Most websites that sell board games also sell accessories like sleeves, so I usually search several different sites and compare the prices for the overall order. I've used Amazon.com, covenantgames.com, maydaygames.com, fantasyflightgames.com, and ebay.com. It depends on what type of sleeve you want. Some are better quality than others (and thus cost more). For games that require a lot of heavy card use, I prefer Dragon Shield sleeves (they fit Dominion-sized cards). They are more expensive, but I've been very satisfied over the years with the quality. For other games that don't require that much handling/shuffling of the cards, I usually go for the cheaper quality ones.
And welcome to the wonderful world of game ownership!
Thanks that's helpful. The Dragon Shield sleeves look quite expensive. But I guess the quality is really good and would be suitable for cards that are frequently used.