If you are using the light detector to switch the flasher circuit, surely a MOSFET would be the best choice? Use a general purpose 2N7000 device, with an RDS-ON of 5 ohms, the voltage drop would be negligable. I think that's best, or does it need to be a P-channel? Similar P-channel devices are avialabe anyhow at a slightly higher cost. If you still want to use a BJT, which should perform fine providing the supply is high enough, use the 3906 for PNP, or 3904 for NPN, cheap and reliable