Most silicone or RTV molds will last for about 20~30 pulls, mold release agent won't wary this drastically.
A Box to make your mold can be done from a variety of materials Wood, Cardboard, Lego Bricks(I use this method), etc.
You will need to watch out for undercuts of course during the mold making process,some undercuts require a 3 or more part mold.
Can't help with places for shopping for materials though, as I get all my stuff from the local Hobby shop.