It’s private agency, not a state or federal government agency, per your post. Receiving federal or state funds does not convert a private entity into a government agency. You did not say what kind of services you were seeking, what the dress code provides, what your violation of the dress code was nor what the other people who were violating dress code did. In particular, was their violation the exact same as yours or not?

Note that neither federal law nor the law of any state prohibits discrimination based on how a person is dressed (although DC law does). In order for discrimination based on dress to be prohibited, the clothing worn must have some connection to something that is protected by law. For example, it is illegal for places of public accommodation to discriminate against persons based on religion. Thus, for example, a place of public accommodation that refused to serve anyone wearing a Christian cross, a yarmulke (a head covering worn by some Jewish Men) or a hajib (a head covering worn by some Islamic women) would violate federal law because it has the effect of refusing service based on religion.

Nothing in federal law or the law of any state prohibits discrimination based on income, wealth, or social status either. Without knowing what these services are, there also may be no law that gives you unrestricted access to these services.