It really depends on the conversion that you are going to use for mesh. I find that many people do it different ways. Plain and simple there are really two ways of doing. Fliping the plate over where the piezo is facing up or leaving it facing down. If you are going to leave it facing down, just go to your local craft store like joanns or micheals and pick up a couple sheets of the 2mm foam.
Now if you are going to flip the reflection plate up where the piezo is facing upward, which personally I like better, because it makes the drum a little more sensitive and responsive. If you are going to do it this way, you can use any low to mediam density foam. The best way I have found to fo it is get your self a 3/4 in sheet of the dense green foam from joanns and cut yourself a few cones out of it. Place some thin double sided tape around the pieze and put your foam cone on it.
Also when it comes to mesh heads there are a lot of different mesh heads out there. By far the most expensive mesh heads are the roland and hart mesh heads. The mid range price is going to be billyblast mesh heads or pintech mesh heads and by far the cheapest are the pearl muffle heads. If you hit your drums harder than the average person then i would suggest the rolands, but remember they do cost almost 40 bucks per head. Honestly I love the pearl single ply muffle heads. They only run about 7 buck each. I keep my snare head really tight because it offers really good rebound that way. I have had my pearl mesh heads for like 6 months and they are steel going strong. I even use the pearl muffle head on my bass drum. With these heads, just make sure you use nylon tiped sticks, because wood tipped stick chip and can shread the heads. Also if you are going to use them on your bass drum, use a wood, rubber, or plastic beater. The felt beaters tend to start pulling on the mesh head after a while weakend and end up making a hole in it.
Billyblast heads are fantastic. They come in three ply which can hold up to the abuse of really heavy hitters. The only down side to 3 ply heads that I see is they arent really good for tensioning them really tight, so the rebound isnt that good. Besides that billyblast heads are just as good as rolands and are half the price at 15 to 25 each. The one thing I do love about billyblast heads and roland mesh heads is that they are white and you cant really see through them, but personally i prefer the feel of the single ply pearl heads, but you can see straight through them.
Just remember a lot of people choose to buy the expensive roland heads, because lets face it, they are roland and most people know roland. Roland makes really great product, but I find most of their stuff to be way over priced. Most of the money you spend on roland products is just for the name. Ive tried all of it because Im a big DIYer and I enjoyt the pearl heads over all. Honestly its going to come down to really what look and feel you like.