It depends. I think most GI doctors who recommend probiotics recommend either VSL 3 or Visibiome, and they are expensive. New research is showing that probiotics are not effective for Crohns. There is a really smart person here called Kiny, that has posted some info about it.
Other than that, if you wanted cheaper options the mainstream one like Culterelle, Florastor, or Align are probably the safer option because they are pretty widely known and respected as being what they say they are.