I don't know if there is an exact replacement available for the OEM underwater transom-mounted bronze bonding electrode (or as originally described, a "disk") on the transom on your 1998 OUTRAGE 17.
A possible suitable replacement may be a GLOMEX RA206 Round Ground Plate. This is a circular plate of 55-mm diameter or 2.165-inch diameter. This product appears to very similar to the OEM disc used by Boston Whaler on boats prior to C.1995. Compare at
https://www.glomex.us/portfolio-items/ra206-rectangular-ground-plate/More at
https://glomex.us/wp-content/uploads/2017/02/Glomex_ground-plates-instruction.pdfOne possible retail source may be from the vendors linked below:
https://www.boatandrvaccessories.com/products/glomex-ra206-2-1-4-round-ground-plateBest advertised price, but currently SOLD OUT.
https://www.tradeinn.com/waveinn/en/glomex-ra206-round-ground-plate/136274345/pThe listed retail price for this item varied significantly with different vendors.
You can use a search on the terms "GLOMEX RA206 Round Ground Plate" and perhaps find other sources.
ASIDE: in most modern boats the circuit that was connected to the bonding electrode connected to the metal components of the fuel system, such as the fuel tank and the fuel fill fitting. In modern Boston Whaler boats, that circuit is connected to the boat's battery negative circuit which is in turn connected to the outboard engine chassis.