depends on where you are buying supplies.. folks who sell to shoe shops or saddle makers will know, but mall shops or craft stores may not. 5 or 6 cord is an older system of thread size and refers to the number of strands twisted together to make the thread. 5 cord is about # 270 to 305 size and 6 is 347 to 405. both are heavy thread used for heavy leather. look for a book called hand sewing leather... tandy used to carry it and probably still has it in their on-line set-up. you will also need an over stitch wheel to mark the spacing of stitches, a groover to cut a channel for the stitches so they won't wear,a stabbing awl to prepunch the stitch holesand beeswax if the thread is not pre-waxed. punches of appropriate size for rivets, copper belt rivets are strongest, tubular, or "splash "rivets , next, then two piece "speed rivets". all will work, but , proper stitching will look and wear better.
e- mail me and i will give you a more detailed answer to any questions.