For me that's part of the fun: Looking for someone who has a particular small interior part. "Men's shopping"!!!
Depends on what you mean with "interior" - dash, instruments, steering wheel and column, pedals, shifter, heating/AC, seats, panels, carpet, insulation.... there's a lot of stuff "inside" the car.
And typical for interior work: It does not matter, how much you got already: there is always another small part that you're missing.
Check the "sponsors-page" here on this site - there are a number of sources.
In my eyes it's always worth to take a look into EBAY, if someone has exactly that particular part you need - usually they are enthusiasts and they are cheap.