Fred, try Hilton's Tent City on Friend St.(right across the street from the Fleetcenter) for Nikwax products.
Also, be careful of the waterproofing products that you "wash" into your gore-tex garments in the washing machine. They do a great job restoring the waterproof nature of the garment, but, they also waterproof the inside which compromises the breathability.
The spray that Pete recommended is by far the best treatment. What you should do is buy the Nikwax washing solution that gently cleans the dirt off the jacket. Then you douse it with the waterproofing spray and throw it in the dryer.
This treatment has helped prolong the life of my Marmot jacket. It's served me well from Alaska to Ireland, and, I have not been gentle with it.