The 'Expressman', or 'Driver', or 'Jehu' or any other sobriquet was first and foremost a 'working man'.
He'd wear clothing as what was available at the local Dry Goods store.
They didn't wear an 'outfit' - nor did they have a 'distinctive mark' that would identify them and their profession.
They wore the same clothes that other working men did - hat, pants, shirt, suspenders, boots, vest, bandanna or tie, coat - and a linen duster or a heavy winter coat as needed. (Buffalo being most common)
Add a heavy coating of dust kicked up by the mules - and you're set.
All you need to document are those items and whatever firearms you'd carry and examples are rife in the Time-Life series 'The Old West' - especially the volume titled 'The Expressmen'.
There's also a book titled - 'U.S. West - The Saga of Wells Fargo' - by Beebe and Clegg - published in 1949.
Your local Library may have a copy or could get it via an Inter-Library Loan.
The 'John Ford Reference Library' often features celluloid stagecoach drivers as 'eccentric' or 'colorful', when in fact they were hard-working men doing their job.
Pick your proposed time frame - then get the most generic men's clothing that you see in dated photos.
If you're in doubt - buy the 'sturdiest', since it'd give the most for your dollar - then as now.
The drivers were well-paid for their skills, since the deliveries depended on their abilities to drive their teams as efficiently and quickly as possible, and that skill had value.
Unlike their counterpart - the Cowboy - they had relatively little predeliction for 'flash', since it was largely unseen due to the dust - so a 'basic' outfit of working man's clothing will be fine.
Good Luck!
Vaya,
Scouts Out!