Sadly there is no warehouse, one tip is to avoid building the Artist's guild until after the you got Museums, and unless you have Cathedrals available. don't built it.
Do Great People cost unit maintenece? not sure, but it comes across as pretty much the same, really. But then again you can use those 2 citizens for something else as you're not spending them on building up your artists.
You find artifacts, which are essentially the same as art as far as slots go - so they go in museums, Louvre, Sistine Chapel, Uffizi, Palace, Parthenon and Hermitage. However, they are different in terms of theming bonuses. Artifacts can only be themed in museums and Louvre, while art can be themed pretty much anywhere.
If you don't have a slot for the artifact, you can only turn it into a landmark, which you would probably want to avoid if you're digging on foreign lands.
Umm, someone suggested a bad thing here. If the dig site is outside of ring 3, turn into a site. It denys the AI a great work!!