Which OOP items? Chaos seems to have been one of the more popular and enduring releases so most items seem pretty easy to get on the secondary market. About the only things that are hard to get in my experience are the E40K Chaos Rhinos/Preds/Land Raiders and some of the classic minor demons. The former are simple enough to replicate with some minor conversions to imperial variants, and the later can be fixed by buying the Warmaster equivalents and cutting up the strips. Warmaster also has pretty close proxies for all the old Greater Deamons.
I'd imagine an EW Droid Controller would make for a decent Warp Palace
. Some of the DRM Andrayada walkers would make ok proxies for Slaanesh knights too.
