Could you be a little more specific? Are you looking for powder coaters in your area? Or what prep work the case of the typewriters needs prior to being powder coated? I assume it's the case that you're talking about and not literally the entire typewriter? 

It would be best to contact a couple of the companies in your area as preparation is often something included in the price. I haven't had a typewriter case powder coated, not because I don't like the idea, but because I've never wanted to spend that kind of money on a machine. Even re-plating is a costly exercise; for example, to have individual nickel key trims re-plated is $6 a piece (in my area), which means a 42-key keyboard would run $252, and that's before you've even addressed all of the machine's control levers or trim pieces on the case.

I could see investing several hundred dollars in restoring a machine that had significant importance, whether it was sentimental or because the model was truly rare, but to spend that kind of money on a run-of-the-mill typewriter doesn't make sense to me, not when you could buy many typewriters for the price.

If you do go the powerder coating route I hope you document the process well and provide us with some before and after photos. I would consider it to be a far more desirable finish than paint, but is it worth the extra cost? That would depend on the typewriter and the importance it holds for you.