One excellent source is industry groups or associations. These organizations exist to further the aims of business owners within a specific industry or field of endeavor. They can provide information regarding a wide variety of topics. Another good source of information are local chambers of commerce and other civic organizations. These groups can provide valuable demographic information regarding the specific geographic market in which you will compete.
Banks are an obvious source of information regarding financial matters.
You're going to have to make numerous assumptions that relate to money, cash flow, interest rates, expenses, etc. Much of what you need to know to make reasonable assumptions can be obtained from lenders. It also provides you with an opportunity to screen potential lenders by seeing the quality of customer service they provide. Potential vendors and suppliers can also be consulted to get information regarding costs, product availability, timing requirements, etc. While there is no substitute for personal experience, you can derive a large benefit by drawing on the experience of those around you.