Where can I get J2000 epoch initial mean longitudes for all planets and celestial bodies?
Take a look at this link. It included Program Codes Listing also (Appendix C). But if you only want the initial mean longitude and the angular veclocity... the appendix C.1 does have them listed at the beginning... in it, the values are L0 and n for each planet... (The n in the listing is in arcsecond, you have to divide it bye 3600 to get your desired degree value.)