AFAIK the AVGA outputs @ 15KHz at all times, yes.
You should be able to get your existing card to output this frequency easily enough. ATI cards are good for that.
If you're using DOS, Mame already has arcade monitor frequencies built in (for standard res). You'll have to specify '-monitor arcade' at the command line.
If you're using Windows, the program Powerstrip as mentioned above will put your card into this mode.