hope this doesn't offend you, but why do you need such a huge amount of grease for? Didn't your underwater housing come with a tube of silicone grease already?
you only need very very little grease for the o-ring in your housing (you should be able to feel it coating the o-ring, but not so much that you can see the layer of grease on the o-ring). Using too much could actually cause the housing to flood, so be careful and don't put too much. anyway, if you really run out, you can get a jar from most dive shops. should be about $10-12 if i remember correctly.