A 10 pack at Amazon is $4.05
http://www.amazon.com/dp/B000FMWPNI/?tag=skimlinks_replacement-20
With my set up I need two for each element and all my surfaces are flat so I am using these:
1/16" thick, $1.87 (+ shipping, free with Prime) for 5. :)...
Mine came out a bit too sour so I'm going to back off on the acid malt next time. I loooove having some "Guinness" on tap at home all the time though....