Construction, specifically concrete construction supply houses usually carry these tubes, which are used for forming concrete columns. Get the supply house to cut your tube rough, that is, longer than you need by, let's say, six inches or so. For an eight or ten inch telescope, one sheet will be plenty. Get at least a dozen. Hot-dipped galvanized box nails work well.
Shingles break easily, so it's a good idea to keep a few extra shingles on hand.
Old belt leather works fine. Visit your stationary store. Three pieces will be used for the lower Rocker Box bearings and four for the Cradle Board bearings. White glue works fine. The back of a cardboard breakfast cereal box works nicely.