How much of a difference does it make when a left handed player uses a ball drilled for right handed. I am left handed and trying to find a used ball but all the available balls are drilled for right handed and hence the question.

Left or right (AFAIK zero chance with a right handed drilled ball) the odds of getting a ball drilled for someone else that feels natural and comfortable is pretty low.  Even if the spans match exactly often the pitch feels funky.  Almost always end up either plugging the thumb or fingers or most often both.  Guess what I would say is more than likely it doesn't matter as will probably have to plug and re-drill anyway.  Getting low scores because too cheap to get a good fit gets old very quickly in my experience. (edit:  actually plenty of horror stories on here as well of people tearing tendons, nerve damage and such by using poor fitting balls)