Here is a library for Fat16/Fat32, Petit FatFS of Elm Chan, translated to PIC18 and MPLAB C18 by Jonas Andersson. The hardware use some level shifters but the library is what we are looking for. Well, it is a start if we want to get rid of the huge MDD-FS library for SD-Card of Microchip. The library is at the end of that page.
Thank you Jonas Andersson!