I buy mine in bulk from US Seal, you basically have to buy approx 3 sets worth.
Barry sells them too, as Wes pointed out
If you decide to go with U.S Seal, here is their web page, check dimensions and order the Viton
http://www.valveseal.com/gas-seal.htm
I think you have a 3/8 valve stem though, verify before you buy
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- 70 Fastback Mustang, 489 cid FE, TKO-600 5 speed, 3.70 9 inch
- 71 F100 shortbed 4x4, 397 cid FE, headers, Street Dominator, 280H, 5 lug Dana 60, 4 speed