You'd be surprised how few senshi have a defined symbol, especially if they aren't a zodiac senshi or a senshi that has an obvious symbol associated with their name. Lots of people just make up symbols, but that involves a lot of creativity. There is hope. There are resources for looking up symbols:

Symbols.com
Religious symbols
Symbol Dictionaries
American Indian Symbols
If all else fails, try a Google image search.

Example:

Sailorma'at uses a stylized ostrich feather done in in gold and lapis lazuli. This symbol can be seen on the front of her fuku.