Another source for zombies is Horrorclix (search Horrorclix zombies on eBay and you will find lots of them for sale). Some comic book stores and gaming stores may sell them, as well. You will have to remove the gaming bases, but they are one of the more inexpensive options.
I have both sets of the Walking Dead metal nano-figs, and they are close enough to O scale to be used if placed carefully.
Andy