Yes protexin is a good one, but very expensive. I just use pink powder which you might find is the most economical type of feed balancer/probiotic on the market and the most effective although it doesn't have pro biotic in it so I am not really sure how it works, as I thought the pre fed the pro!! Do they go hungry then??
This contains pre-biotics and pro-biotics but is also extremely high in anti-oxidents, oxidative stress has a massive impact on your horses health and soundness.
have a look at the site and recommend watching the vids.
Whatever product you decide to go for - make sure that the amount in the product is actually worth feeding and not just a token amount within the ingredients so that it can be sold "with Pro/Prebiotics" on the label.
Kate x