Can you purchase 1 plug wire anywhere. Went to the dealership and they said you have to purchase all 4 for that side. Getting a cylinder 8 misfire code. Figured would try changing the wire and plug on that cylinder. This is on my father-in-laws 97 4.6.

 

You're lucky that you can just get 4, I've never found less than a full set anywhere ever. I know, it sucks!

 

Swap your #8 wire with another wire on the same engine, then run it for a while and recheck your codes. If the other cylinder is misfiring then it's the wire. If #8 is still misfiring than it isn't the wire.