Slips are hard to find. I think JC Penney has the best selection of slips, from various manufacturers. Slips usually come in nude, white or black and they come in a different lengths. Walmart may carry them too but they would have less of a variety. I think I have seen them in a Maidenform or a Vanity Fair Outlet store too.
What you want to buy depends upon if the top is sheer (a full slip) or just the bottom (a half slip) and the type of straps on the dress. You may want to bring one of your dresses with you when you go to try them on. For longer dresses I think a slip that goes to a few inches below knee looks best.
Good luck!