There are lots of useful tips given to you already.
I've used this stuff for years. I find it's perfect for everything.
http://www.gooutdoors.co.uk/lifeven...g-l8fCewy9wxlrTLf5RhVRoC567w_wcB&gclsrc=aw.ds
I don't know if it's available SJPDP and I don't recall seeing it in any shops along the Frances route.
I don't know where you are and I don't want to complicate your planning, so I offer this suggestion with some hesitancy. If you have pre-booked accommodation in SJPDP, you could consider sending some to yourself c/o your accommodation, or if you chose to buy it on line, use your accommodation as the delivery address.
Buen (lots of soapy-bubbles) Camino