I've named my shop Ten-Forward, so I want to have a HA of Guinan to greet those who enter the shop. But finding items that resemble something Guinan could wear is excessively difficult.

Guinan is played by Whoopi Goldberg, and known for her long robes and excessively large hats. Here's some example images:

I'd appreciate any help you guys might be able to provide :)

hats and robes are a real loss on this site. However, with some layering you might be able to work with some of the graduation gowns, maybe with some nice scarves and belts or such. OR, conversely, there were some long flowing skirts recently, maybe with some large drapy top over them or something you can get that effect. As for big glorious african-inspired hats, we have a dearth :/ There are the sahiric head wrap things, and more recently the but there isn't much else :/ Sadly. Good luck!

Oh, hey, my shop is called Ten-Forward too.

Maybe these would work? I'm thinking you will have to layer several items to get anything close to what Guinan wears. You could also try later episodes with her, she changed her outfits a little.