You get paint from a paint distributor, not the dealer. There are tons of variables in this. To be honest, if you don't know where to get paint codes or paint, will you be able to paint the car yourself? It is much harder to do a good job than most know. Practice makes perfect for this. I would know, I am ASE Master Tech certified.
To specifically answer your questions, look at the data sticker in the door jamb. It will have a line that says paint and a two letter code. For example black would be UA. Talk with a few different body shops about their paint distributor. They may use PPG, DuPont, Sherwin-Williams, R-M, Glasurit, etc. Professional paint is all very similar in my opinion. Most shops use one or the other based on their relationship with the distributor and not the paint. Now, there are tons of fanatics out there about their brands being best, and everyone has an opinion. Cost is not always an indicator of performance. Contact a few paint distributors in your area and ask questions. You cannot ask enough questions before purchasing the paint.
How much paint to buy is a loaded question. Are you painting the jambs and undersides of hood/trunk/etc? Are you doing a complete color change? Are your painting skills and equipment good enough to use minimal product? Most people would say to get a gallon and call it good. I have seen this be more than enough with extra for touch ups. I have also seen people run out in the middle and scramble to get more. The problem with running out is now you have two separate mixes and the chance of slight color variation increases. It appears you are painting with the original color. A gallon of basecoat should be fine. You will also need to remember that you also need to purchase clearcoat as well as catalysts, hardeners, reducers, etc. Depending on the brand, you may also need to purchase sealer that would be sprayed before any color coat would be applied.
