Diodes to isolate two circuits?

Hi, I am wiring a custom car from scratch. I have succeeded with no problems, but I need some help about one thing please.    I have the turn signal circuit working. I have a column mounted turn signal switch and one flasher. This is working perfectly. I added a second flasher, using a separate on/off switch mounted under the dash. I wired this to the same turn signal lights. The problem I have now is that the turn signals are backfeeding and when the column mounted switch is turn on all lights flash. Ditto on the switch side. So, I guess I need to use a diode to isolate the two circuits feeding the same lights. Would somebody please tell me what kind of diode I need? Are these readily available at Radio Shack? Sorry to sound so ignorant, I just want to make my flasher circuit work. Thank you all very much for any help... regards, joe