Obtaining substrate is always a perplexing question, especially when you first start the hobby. Your options are:
1. Buy from bugsincyberspace.com. Easiest, high quality, best choice for someone new. Yes, it cost but unless you opt for the other two options, this is your best choice. BIC's pricing is very good, the problem is the shipping. There is just no way to get around shipping a big bag of something. So it seems expensive but it's really the shipping cost.
2. Make your own flake soil by fermenting Oak BBQ pellets. There are many threads discussing this and the process. Besides the amount of work involved, it generally takes at least 3 months. So if you need the sub right away, this is not an option.
3. Go out into a forest and find decomposed oak (white rot) or other decomposed hard wood. This can be good if you are in an area that has dead oak trees or logs, etc. However, if you don't live in an area that is conducive to finding decomposed oak, this is can be super difficult and time consuming. ie: You drive for hours and then spend hours looking for decomposed wood, gather it, etc. For a lot of people this is just not practical. For those that have access to it, like a friend I know that has generously sent me some when I run low on sub, you are blessed!
These are the options that I am aware of. If there are other suggestions, please post them. Too bad we don't have stores here in the US like Japan that we can just drop by and pick up some substrate anytime we feel like it.