Who makes them, where do I find them, who has the best?

I was looking for a Surefire mount, and Aimpoint or Reflex mount.  Handguard funriture changes are what I had in mind.

Thanks.

Do the top receiver covers with built-in rails work on Krink's?

Cheaper-Than-Dirt had single rail covers for $20 and three rail covers for $40.

I'll look at those, but since the top cover likely does not mount to the same exact position every time I would think that the sight would be off every time you remove the cover.

There used to be a top cover with mount that had screws you could tighten to counter the "wiggle" you get.

You still might have to do a minor adjustment every time you popped the cover off.

Will regular AK siderail mounted optics interfere with the rear sight on the top cover?

I'm going to go out on a limb here and say "NO" a side rail mounted Kobra or PK-AS will not interfere with the rear sights of a "krink"  I have seen Kobra sights installed on Bulgarian AKS74U.  
You will not be able to mount a after market top cover with optic mounts on a Krink because they all have hinged top covers.  Besides.....you would never want to "Bastardise" a beautiful krink with a aftermarket turd like that......
Even though they say that a AKS74U didn't come with a scope rail installed I am thinking very hard about putting a Kobra on a Bulgy kit that I have.  Besides.....I have a extra Kobra just laying around collecting dust.....