For what it's worth, when I got my K4 used, the header bolts were all rusted out and crusty, and quite hard to unscrew. I bought all 8 new bolts OEM, and yeah it was a lot of money. But I rest easy knowing that the bolts holding basically the hottest part of the engine on are the right spec.
I am not sure if these exhaust bolts are something special, or coated special, or a certain type of steel that can handle those extreme temperature gradients or whatever, but I didn't want to find out some random bolts I got weren't good enough, and then have stripped/galled/seized bolts in my cylinder head.
Does potentially having stripped/galled/seized bolts in your cylinder head seem worth saving the $20? You could probably get some random black oxide bolt and make it work and it would probably be fine, but this was one instance where I didn't want to experiment.
Sounds like you just need one bolt. I would advise you just buy the factory one from a legit seller, and if you need any other factory bolts or parts, just order those too while you're at it. Factory bolts are always pricey, but there is a lot of coatings, metallurgy, grade/ratings, and tolerances baked into those bolts. It's hard to stomach paying that much for a bolt, but it's usually not just a bolt.
-Mike