Depends on how much you want to spend? I got mine from Matt, Octo Aquatics. He makes really nice lids. It's polycarbonate, so it doesn't get smokey or foggy like basic acrylic does.
I'll second Matt (OctoAquatic) workmanship. They are not the cheapest on the market (but not overpriced for what you get at all). Working with him was a treat, meaning that you can get your lid designed exactly how you want it (doors, cord openings, etc).
Personally, I have 2 lids (1 for each of my tanks) as well as 4 other of his goodies (dosing tube holders, mushroom box, fish container, and nori clip) all are amazing.
That being said, if you need a cheaper version, the RedSea one that DEW linked, works well too. That is the one I had on my tanks before getting Matt's.