There may be other blue foam sheets, but I will tell you about the sheets I have. I have used these and been happy with the results. I use them in my cartridges when I'm not dripping.
The blue foam sheets I am talking about come from the pet store, specifically the fish department. It is used in aquariums. I purchased mine at Pet Smart. They cost around $6 to $8, if I remember correctly. One packet will last you a really Long Long LONG time!
Marineland Bonded Foam Sleeve - Type U, 3 sleeves in the pack. Fits Hot Magnum Canister Filters.
Here is a link to a Blue Foam Modification for Cartridges:
http://www.e-cigarette-forum.com/forum/cartridge-mods/58690-b-f-d-cart-mod.html
There are other
threads here in the forum that discuss other blue foam options. You would be able to find them in a search for blue foam.
As I said, there may be other blue foam products. This is the one I am familiar with.
Paula