BCW has postcard sleeves and toploaders. Still a bit larger than a 3x5, but not quite so oversized as a 4x6 topload. They have 3x5 toploads too, but as with the others you mentioned, I think they would be too tight to allow for a sleeve.
http://www.bcwsupplies.com/products/...16-x-5-3-4.htm
http://www.bcwsupplies.com/products/...oad-holder.htm
Another alternative is a 3.5x5 topload and still using your standard 3x5 sleeves.
http://www.bcwsupplies.com/products/...oad-holder.htm
Just depends on how picky you are about the edge of the sleeve going to the edge of the topload vs. the amount of empty space around your card.
Also, cardboardgold.com has the CardSaver IV's, but only by the case (don't know how many index cards you're needing to house):
http://www.cardboardgold.com/card-savers.html