I am not sure what kind of plant leaf you are interested in.
For Arabidopsis leaf, you may check out the following protocol.
Flury et al. (2013). MAPK Phosphorylation Assay with Leaf Disks of Arabidopsis. Bio-protocol 3(19):http://www.bio-protocol.org/wenzhang.aspx?id=929
For tobacco leaf,
Husk et al. (2014). Monoclonal Antibody Purification (Nicotiana benthamiana Plants).Bio-protocol 4(2):http://www.bio-protocol.org/wenzhang.aspx?id=1034
Both above protocols have details about how to extract proteins from leaves including the recipes of the extraction buffers.Hope you would find them helpful.