what are some good ones a lvl 87 could kill?

any monster that drops gems will drop half keys and dragon left halves. I don't think any monsters are better than others for this.

i get most of my half keys from randoms.. lol.

Fire giants and ankous are decent half key droppers for me. Basically anything that drops gems drops half keys.

I've heard from friends that they go to the Other Worldly Beings or whatever in Zanaris to get half keys, but I've never tried this. I get most of my half keys from Fire Giants, Metal Dragons, randoms, and Greater Demons. I occasionally get them from others though.