Try Lowes at the link below or short name at the bottom. I have not used the one in the link. I like an aluminum Disc I see the Yardmen and Park Crews use with 4 strings threaded-in that I was told is from Stihl suppliers.

Meanwhile I have drilled holes in my feeding head to accept the largest/strongest line I can find. Visualize drilling 4 equal holes or using the two existing. Pre cutting line and tying knots to allow 12â Etc to extent. It will wear and balance but start close to balanced. The Line Still Breaks If Your Trimming Is Like Mine.

http://www.lowes.com/pd_29767-42694-16738_0__?productId=3382164&Ntt=string+trimmer+heads&pl=1ätURL=%3FNtt%3Dstring%2Btrimmer%2Bheads&facetInfo=

Here is a link that might be useful: trimmer heas