Give the forum a search, there are many threads with suggestions.
I purchased this Hard Tri-fold almost a year ago with a $30 coupon. I Like the way it fits and looks and is holding up well considering I park outside.
Do you want one that "flips up" (folds up all three sections so it stands up)? A lot of "tri-folds" are actually "bi-folds" where section closest to the cab is always flat, blocking the bed. The better ones are the "flip-up" type that don't block the bed by more than a few inches but they cost more.
The better known flip-ups are from BakFlip (the MX4) and Rough Country, I have the Rough Country which is lightweight and cannot have a heavy load put on top (45lb weight limit) where the BakFlip can hold about 400lb of evenly distributed load. The Rough Country is less expensive.
I did replace the clamps that came with mine so that I could use the cargo rail, many of the tonneau covers come with clamps that are too big and block it. The link that @Shawshi gave also has a model that is a flip-up for more money. But the clamping system on the Tomoka to lock down the panels is not as nice as on my RC or the BakFlip MX4, they jut downwards into the cargo area (you can see in the customer review images) where the RC does not.