Waterproofing pizza oven

Hello - I've finished building a cob pizza oven, but now I need to put some sort of coating to waterproof. I've read a lot about lime plaster, but I haven't been able to locate the builders hydrated lime powder (as opposed to that used for gardening). Can anyone help me out...either locating a supplier or offering an alternative? Thanks in advance

find a local brick dealer, they will either carry it or know who the supplier is.