Can't seem to figure out how the flasher mounts under dash. Is there a bracket that this mounts to?

yes. A metal spring clip, attached with a phillps screw that comes up thru the dash, near the lower edge of the main dash, between the guage carrier and the raised center section. clips in with the wires pointing forward.

68 asm manual page 163 UPC 12 sheet B4

Anyone know where I can get my hands on the spring clip?

looks kinda like the old fashioned flashlight holder on your handle bars of your bike, only smaller.
Im betting alot of cars used that type of clip so maybe a trip to the boneyard?
Ive not seen them in catalogs.