Chevy T5 mech speedo drive can be got from Real Steel, make sure you ask for and get all the bits you need - retainer/gear/bolts. 01895 440505. You could ask them about electronic speedo transducers, or try Speedy Cables 01639 732213, ask for Andy Barraclough.
Other way to get an electronic speedo is via magnets/hall effect transducers on the propshaft somewhere. Sure I saw a post about that recently - try a search.