Unfortunately you need to have received training from the certificate holder you're going to work for (ground and flight) or from a approved training vendor (Simuflite, Flight Safety, etc...) for that 135 company.
A lot of the training and testing can be conducted by a vendor, but part of the testing (135.293(a) 1 & 4 thru 8) that is company specific, regs, etc... that most vendors don't provide training for, must be done by either the certificate holder or the FAA.
Good Luck
Shawn