Welcome!
There are a few sources that you can go with. Unfortunately there are none i can recommend that are in the US, its slim pickings on this side of the pond.
I have used mainly Axel Gerstl for my 500 parts, they are located in Germany, and ship to Canada/USA but do not offer express and typically takes about 4 weeks to arrive.
There are many UK suppliers that ship express, im sure others will chime in with suggestions, there are a few reliable sellers. Ive used Ricambio which has shipped DHL and i got things within a week.
Unfortunately there is a price to be paid for shipping parts over, but they have all been reliable sources. Quality of parts vary significantly for these cars, so keep in mind when shopping.
Dont even bother wasting your time on ebay.it incase you need something special. I spent alot of time and effort with translations and such trying to hunt down stuff. Mainly out of necessity due to something unobtainable elsewhere, or the price was too good to pass up.