I would start with a part time sales person in the shop to make sure that every thing runs well but at the same time not interfere with your already established reputation for repairs - service.
Again as others have suggested already you can contact your local school at whatever level you require if only for "Work Experience" as they call it over here and after you have had a few come through the doors you will get a very good idea on who is best to suit your present needs.
While I don't know about these schemes where you live over here the business who participate are paid for allowing these schools students into the place so if that is what happens where you are it will mean some money in your pocket for any inconvenience.
When you finally find some one to work part time which no doubt you already need I would follow the above recommendations about supplying product at cost as it works better than just a minor discount where the people feel ripped off by the "Rich Business Owner" which I'm sure none of us actually are because if we where we wouldn't be working in this field any more anyway.
Failing the "Work Experience" model if that isn't available where you are there will always be some ones son/daughter who will fit the bill and want to work if only for a short time to gain experience but there are draw backs in this area as well as these young kids tend to think they know it all and you being the "Dinosaur" that you are will only benefit from "their vast knowledge." But even this is great experience for you as a budding "Manager" as it will teach you "People Skills" which very few people actually have in the first place.
Then if all else fails you can always approach the local unemployment center to provide part time staff from the "Long Term Unemployed" as there are some great incentives for business to help train these people but again that will mean following the Laws to the letter about employing people in your area but with enough finical support it may prove useful as it will put you on the right track so you know exactly what to expect when you start employing people full time. However there are generally time limits on these schemes and you do not want to be perceived as using the system to your advantage by keeping the person there while you are reaping the rewards and then getting rid of them when the rewards cease and replace them with some one else who will bring in the money again if you do this for any length of time you will begin to rely on it and be running a "BAD BUSINESS" which can not support the staff that are necessary for its smooth operation. You will learn very fast if the person who you pick is right for the job and it is more a reflection on your self rather than the people who end up working for you as you where responsible for choosing them in the first place.
From my experience however once you start to employ people your time might be more freely available but the responsibility of keeping a job for those people is enormous and your take home money drops until you have expanded the business to the stage where it can support what ever number of employees that are working there and the "Paper Work only gets bigger" and more time consuming so you have even less time to go out and make the money that you originally started the business for in the first place.
I hope that is of some help to you.
Col