I would suggest cold calling, I think it is the most cost effective way to get ahold of sellers. Direct mail can get expensive, especially if you are just starting out and aren't sending out to super niche list.
In regards to getting list, you can never go wrong with driving for dollars. You will see other properties that your competition won't. Get the app Land Glide ($100/year) it will allow you to tag properties you see and build a list. Then you can export your list into an excel format with all of the relevant info (property address, mailing address, owner name, etc).
For skip tracing, you don't need to get too complicate with it. There are a number of sites that will skip trace your list but there are also free alternatives like true peoples search. It might not be the most accurate data but it is free and will get you started. Once you get a feel for the game, then you can scale up.
Feel free to reach out! I would love to help any way I can.
-BA