You are going to have an easier time finding all the things you need at online suppliers...the drawback being the wait time for shipment...but the prices and availability will be much easier.

The West System Epoxy is what I use for making parts - but the extra strength is not really needed for a mold and is considerably more expensive than a polyester tooling resin. Second the nomination for the gelcoat - it's not recommended to use with epoxy resins.

You can use a standard gelcoat but a "tooling" gel coat is designed for making molds (tools). Then use a laminating polyester resin to build up the structure around the gelcoated plug. Chances are that the gel coat you purchased at West Marine has a wax additive to allow it to cure in open air (the wax floats to the surface and seals the gel coat off). Gel coat will not cure when in contact with air (which is fixed when you are making a mold by constructing the resin and fiberglass around the gel coated plug sealing off the gelcoat and making a great bond). To be sure, check the can of that gel coat to see if it is "air inhibited" - if it is, then it does NOT have the wax. if it has wax, it could affect the result of your mold surface.

http://www.shopmaninc.com/index.html

http://www.fiberglasssupply.com/

http://www.fibreglast.com/

http://www.aircraftspruce.com/