I doubt that you will find any, and your jurisdiction should not allow it if you did. The tank has to be water tight for its entire life, and I dont think you can build one out of block that will not crack and leak sometime. The time and expense of pouring the bottom, building and reinforcing the walls and then pouring the lid with the required access holes, (and then removing the internal uspports for it), would seem to be more than the cost of purchasing one. Fiberglss tanks are very inexpensive if your locality allows them.: I have been surching the net trying to find plans to build a cement block septic tank