It is standard to choose pantone colors from the swatch booklet, as the colors on the monitor are not necessarily showing accurately due to monitor calibration and other settings. You would need to purchase pantone books-they can be quite expensive, but are often available on Amazon used. There are a number of books-coated and uncoated being the most common though there is also metallics etc. Ideally, you and the client would use the swatch book to pick colors, as that is the only way to know how the color will really print. Then you choose that color by number in the software, and often will see that on the monitor it does not look the same as the printed version. If they are getting their design printed on a home printer or somewhere that does not have the capability of printing with pantone colors, then the print would show in the closest cmyk match. But if they are actually printing with pantones, it will be the exact colors from the pantone book, including coatings and finishes shown in the books. If you need to accurately match a cmyk color to pantone, the most accurate wait is to get a good print on a laser printer (or the best print you can get) to accurately see the CMYK color, and then to compare it to the pantone book in order to get the closest match. Matching on the monitor is not accurate enough for jobs where accuracy is crucial.