All funding agencies identify specific areas in which they provide support and a proposal will not be considered if it is not within the specified area of interest regardless of how well it is written. Identification of a source whose interests match your own will be a major factor in submitting a successful proposal. Some agencies have funding programs which are highly competitive and others fund a relatively high proportion of requests. However, do not be deterred from submitting to more competitive programs if you have a project that is a good fit with their interests. The largest source of grant funds overall is the federal government.
Most Government programs have clear deadlines so advance planning is essential.
Since many initiatives are run on an annual basis, reviewing program requirements and deadlines for the previous fiscal year can provide a guideline on what to expect so you can plan accordingly. Information on most Government programs is available on agency web sites so you should be able to identify deadlines in your area of interest. Foundations, corporations, and various professional associations are also sources of funds. They generally have very specific program areas in which they are interested. There can also be specific restrictions related to issues such as the type of organization they are willing to fund or the geographic area in which you must be located.