You can find artifacts that were created in a previous fortress. Putting artifacts in bins made of heavy metals such as lead, gold and platinum will prevent the items from scattering, which happens when a fortress crumbles or is abandoned. There are no other sources of artifact equipment other than past fortresses.
It's unrealistic to equip even a single adventurer with all-artifact gear, but masterwork gear is almost as good and is significantly easier and faster to create. Similarly, store it in heavy bins if you want to be able to find it with an adventurer.
To store items in particular bins, create a stockpile that will only accept artifact quality items, forbid all bins but the ones made of suitable metals (make sure there are some empty bins of those types), and designate the stockpile to take from the stockpiles where your artifacts are stored.