Because under-cabinet lighting is such a standard in homes, it is available in many kinds of stores. The sales associates in those stores will probably be very informed about the lighting systems and will be able to happily assist you if you have any questions about the lighting units before and after you purchase them.
However, if you find that you have questions or concerns about under-cabinet lighting that the customer service representatives at the store cannot address, there are also many other places that you will be able to find answers. The internet is an excellent source of information regarding under-cabinet lighting, and various sites cover the spectrum of lighting questions. You could also consult an electrician if the question is installation or power related; or you can contact an individual salesperson of under-cabinet lighting, who will probably be even more proficient in his or her field than some store sales associates. Whatever method you choose to use, you are sure to have all of your needs met when finding information on under-cabinet lighting.