Retrieving the sentinel L1C 20m grid

If you mean the Sentinel-2 tiling grid you can find it on ESA pages.

Direct link to kml file: https://sentinel.esa.int/documents/247904/1955685/S2A_OPER_GIP_TILPAR_MPC__20151209T095117_V20150622T000000_21000101T000000_B00.kml

If you want to know if it is possible to somehow calculate this grid from scratch, I do not know.