I am not sure if this has been mentioned anywhere in the forums yet, although I feel as though it probably has, but if you haven’t gone to McMaster-Carr website yet then you may want to do that.
They have pretty much anything I have ever wanted from screws/bolts/nuts to aluminum extrusions, pulleys, linear rails and tons of other stuff that you can get lost in.
I did a search just now on the forums here and found some instances where it has been referenced. Thought I would bring it up again for those who find it difficult to find hardware locally or online. Serves both the metric and imperial worlds in most cases.