Shimano M355 brake handle parts
The circlip and master cylinder are not shown in the Expanded View or Dealer Manual. I looked up EV-BL-M975 for a lever I own which I know uses a circlip and that document doesn't show it either.

I don't think Shimano intend for the levers' hydraulic internals to be serviceable, whereas many of the reservoir covers and seals, lever blades, pivots and other parts are listed with part numbers.

A machine shop may be able to identify a compatible circlip by examining the other lever. As Shimano's hydraulic brakes seem to be compatible throughout the range, I expect the cylinder diameter will be identical and the same circlip will be used.

If the circlip was pulled out, it's likely the groove will be damaged. Personally, I would buy a used Shimano hydraulic brake lever (any model should work but it's best to get one which uses the same fitting size) and fit that.