If you are interested in AC over water you can try the one included in the C2RCC. It is included in SNAP 6 BETA version (http://step.esa.int/main/download/)
There is also iCOR. An AC for Sentinel-2 and Landsat 8.
You will need to follow the manual for the installation instructions.
There is also ACOLITE. This tool is not related to SNAP. It does the AC also only over water.
Finally, you can get already surface reflectances from USGS (https://landsat.usgs.gov/landsat-surface-reflectance-high-level-data-products). But these products are not yet supported in SNAP.