If you live in the USA you can purchase a Bi-concave lens from Edmund Optics . A Plano-concave will also work. You should also know what the focal length of the original lens is.I'd write to either Wicked lasers or Leadlight directly and ask.
I suggest you buy a Plano-convex, cause the selection is larger. AR coated lens would be good too. I think about 4% of the light is lost per surface with non Ar coated lenses, in other words the light never gets out.
You may need to re-collimate.There is info I think at Wicked Lasers.
This is only a guess but I suspect that the negative focal length lens has a focal length around 3mm.
Another link which is useful Gaussian beam propagation
