This question would get better traffic in the garage section.
There are many ways to look for the truck you want. My vote is all of the above. Especially since you seem to be looking for something real specific. 87 only, or will 87-91 work? Are you trying to recreate something you used to own? What kind of price are you willing to spend? And condition plays a big role in price also.
Keep checking around here at CK5, sometimes real nice K5s get posted in the various sections of the forum here, usually under a craigslist find or ebay find. Sometimes members will hunt for their dream truck, even though they are not in the market for one, and post up some good finds on this site so the rest of us can drool too.
Remember, GM stopped making these things 20 years ago. The more specific your search criteria is, the harder and longer the search will be. My advice to you is open up the search criteria a bit, you may not find exactly what you're looking for, but you could get close enough. And the good thing about that is, these trucks are like Leggos, if you don't like what you have just add or remove untill you've built what you want. Real nice original survivors are out there, they do exist. Good luck with your search!