Can I use a universal dimmer instead of a Mark 10 fluorescent dimmer? They are expensive!

No. Unless you have a special ballast explkcitely designed to work with a "standard dimmer". Except the "dimmable" selfballasted CFL, I'm not aware about any being commercially made, although it is technically possible.

What you may find as a "fluorescent dimmer" is not a real stand alone dimmer, but a (user interface control) component of some dimmable fluorescent ballast system, usually proprietary to work only with the specific "ballasts" of the given manufacturer. Even when the system allowed multiple lamps to be tied to a single common "dimmer" control box.
Only recently the "1..10V" was the somewhat standardized dimming control interface where you may combine components of various brands, but before that it was practically all proprietary only.

I know Sylvania made F32T8 dimming ballasts that could work with standard incandescent dimmers, as well as 0-10V controls.

I haved use rotory fan speed controller to dim preheat chokes and RS ballast that have independant drive from the cathode heating coils.