My suggestion is a good, old-fashioned brainstorming session. Grab a pad of paper, some sharp pencils, and go for it. Start with a list of words to get get the juices flowing: dollar, coin, bill, government, etc. Think about features and benefits the client offers. How can those be conveyed thru an icon?
Use a thesaurus and google words. You can go to Shutterstock or the like to search for ideas…
Since it’s taxes it will probably be really straight forward.
Look at logos by their competitors and the largest companies in the world that do the same sort of work.