I'd guess you can get Maxamps to do a custom pac for you sinc they sell 5S1P, but at that price, I'd build my own.

I bought a DeWalt 36v batt and its real easy to make 2 5S1P cells as they come out of the DeWalt box. I'm going to use them as a 5S2P, but it will be two pacs so I can balance the weight easier - pac on each side.

I'm waiting on a set of balance taps and mine will be done except form some shrink. The two 5S1P pack will have cost me $115- I got mine from Amazon.com (free shipping) and you can do better off ebay, but it takes some watching. My next a123s will come from ebay.