If you go to a local shop, make sure they know something about cycle seats and about sitting upright in one position for any length of time. Many upholstery shops use foam that is WAYYY to soft and compressible for a cycle seat. All of the better seats use a very dense foam that does not pack down over time. Those soft, pillowy sofa cushions are only good on the sofa, no place for them on a bike.
The shop must also know something about shaping the seat into a slight dish shape, concave and not convex, as a dish shape distributes the "load" over more of the outside of the butt instead of in the center like many cycle rear seats do.
I made a custom seat for my 94 BMW RS, after I studied seat design at DIYMotorcycleseat.com. I added almost two inches of very high grade firm foam under the passenger seat for my gal, then formed the seat into a dish shape, with slanted relief at the front/sides for her thighs. She said it is very good, except over time, sitting puts pressure on the "sitting bones", those little projections on the bottom edge of the pelvis. Look up "sitting bones" to find more detail.
The front portion of my seat I also built up and reshaped, including an airflow slot up through the seat pan. The seat I'm designing for my next BMW R1200RS project will have a center dropped section for airflow, and both the front and rear sections will have gel pads under the vinyl for the sitting bones. The same shop where I bought the foam ($50 worth) is where I had them make and install the seat covers on the foam I shaped.