1. Where is the best place to buy dry ferts?
I have bought from GLA and Nilocg. I have had good experiences with them both. Nilcog has mentioned a Black Friday sale coming up. Might be good to get on his mailing list and save a few %. I don't think you will be disappointed with either shop.
2. What exactly do I buy? (Im a total beginner that learns very fast)
I have mostly only done the EI method. As you fiddle with your levels you might find that you want to try some different things, but starter items would be:
GH Booster if your GH is low
Iron Gluconate if you want to try different iron recipes
Mn because some people dose this extra and have good results
MgSO4- available from any store as Epsom salts. I need additional Mg in my tank above and beyond the GH booster. You might not need this.
I wanted to try Nilocg's premade ferts just for fun. I know they are liquid, but they are much more concentrated than Seachem products. I have a small tank now. I know not as economical for you, but might be easier to get started with. They are all dosed 1 squirt per 10 gallons. (I think)
3. How often am I dosing?
For both you usually dose 3 times a week with a 50% water change once a week. Your methods may vary.
Good luck. Going high tech and learning about dosing can be very frustrating because everyone's tank is a little different. You'll get it though.