The sleeves are a real pain to find, as are the top loaders which snugly fit 3X5s. Try
www.shopbck.com and ask them. They're really nice guys from the Chicago area. They set up at the National which is where I've purchased both types from them. They've told me that they only charge actual shipping costs should you wish to have some shipped.
Whoever you're dealing with when buying either of these, it's always best to ask them to try sliding an index card into the sleeve/top loader to make sure the fit is correct. It's so common to end up with ones that are a bit too tall on the height or just a touch short lengthwise, and nobody wants those.
You may also wish to scout around on philatelist supply websites if you're OK with those glassine type of envelopes which also fit 3X5s. They're a bit too cloudy and papery for my taste, but if your main goal is to protect rather than display, these may prove to be your cheapest option.