Depends on the size of ya dogs neck but I got Dex a cyclists arm band from tesco- with four LED's on it that either flash or are static for about three quid, Dex has a fat next though so it was a bit tight round, but is really good attached to his harness.......
Second the Tesco flashing armbands, cheap enough that when the batteries give out you can just replace them. I did meet 2 huskies in our local field yesterday, one with a blue flashing collar and the other wearing red, no idea where they got them but very effective.
I paid almost £20 for a snazzy Ruffwear beacon flashing clip-on light thing; the £1 bone-shaped pound shop version is just as bright. Saves faffing with swapping collars, just clips on.
I have these for the ridgeback and for one of the pointer crosses as he has a short coat, did get one for the fluffy one as well, but you can't see it very well on her.
I got mine from www.equestrianclearance.com
Great fun taking videos of the playing with them turned on because they flash it's like being at a rave!!!