Hi Lucylou. I wonder if you might be worrying unnecessarily? Once in about every 10 poos, a chicken does what is called a caecal poo, which is often quite frothy and also runny, colour varying from sulphur yellow to darker brown. This is entirely normal, something to do with the way their insides work, I'm not sure what exactly. You can see some pretty gross images here
http://www.the-chicken-chick.com/2012/02/whats-scoop-on-chicken-poop-digestive.html
Which may put your mind at rest. Just Google Chickens caecal poo for more images if needed.
If a hen truly has diarrhoea it will stick to her vent and the feathers at the back end and won't just come and go, it will happen all the time and it's often green, indicating an intestinal infection. Probiotics may help with this but aren't necessary otherwise. So, are your chickens' bums clean?
If you want Beryls you can get it from The Chicken Vet site see
http://www.chickenvet.co.uk/shop/products/index.aspx/category/nutritional-supplements-5
But as you'll see its not cheap and I'm not convinced that it's necessary, especially if you are not sure of a diagnosis. My hens seem to stay healthy and happy for years on end without any supplements whatsoever, just routine care really, and of course good healthy stock to start with.
By the way, have they had a course of Flubenvet since you got them? They will need worming if they haven't been done. Worms can cause diarrhoea as one of the symptoms but I don't suppose yours are at that stage yet even if not wormed recently.