I've got an EDT5, and I like it alot. I payed quite a bit for it new from the dealer, but you could probably find one on ebay or something like that.
Other than that, my dealer had a Briggs & Stratton tach that you had to wrap the wire around the plug lead, and then attach to a ground, for like $40 or $50. You're paying for the conveninece of just pointing the EDT5 at the saw, which I think is worth it.
Take care all,
Jeff