Morgan do use bought in parts such as brakes etc where it's bonkers not to.
However, there are so many unique parts that are made in- house that are part of the originality of design that it would be uneconomic for anyone to try making copies of. There are specialists who have developed parts - Heart of England/ Librands/Mulfab etc who offer enhancements or alternatives but there are many parts only available from Morgan. Their machine shop is full of them!
Where bought in parts are used then it is possible to source elsewhere e.g. Ford engine parts, Rover engine parts. Electrical parts can also be outsourced once they are properly identified.
Where you actually go depends on you. Do you want every invoice to show Morgan on it?
See my post earlier re German dealer and customers paying over the odds for tyres but getting the dealers name on the invoice!
Contacting a dealer can often be the best option with Morgan as they will have the ability to ID the parts needed properly or source alternatives if appropriate.
Mainstream cars are more likely to have companies make pattern parts as the market for them will be big enough to justify, unlike the small volume uniqueness of Morgan. Who would try to copy Morgan hubs? Ash frames? There simply isn't the market.
I'm glad I don't have a Ferrari/ Lamborghini/Pagani/Bugatti!
Now there's some scope to empty the bank account!