Well if you find it impossible to find the 99-00 you can get on of these bad boys for 135 bucks and a 96-98 will plug right in. I found this with a super quick search. There may be cheaper ones.
JHPUSA ECU Jumper Harness OBD2b to OBD2a - ECU Jumper Harness - JHPUSA
So how much did your ultra gauge and mpgino cost? Are you happy with it? Was it easy to set up?