Using Replace Field Contents to update the field to have consistent contents across the entire table wouldn't be a bad idea. Using a value list for input is also a good way to prevent inconsistent data entry in the first place.
used as find criteria will find both USDA and U.S.D.A, but may also find other records with data such as United States Department of Aviation
Using your data as it currently exists, your best bet would be a find request with two requests:
Enter find mode
put USDA in the field
Select new request from the requests menu
enter U.S.D.A and perform the find.
A third alternative would be to perform the find with criteria entered into a calculaton field where punctuation and spaces are filtered out. That's a "special case" solution best used from a scripted find, but it would work.