Go for good quality apple cider vinegar - particularly with the "mother" still obvious (it gives it a cloudy webby like appearance).
I initially purchased it from a health food store for both myself and the dogs - the taste was terrible! However, I now buy it from a place called crossgates farm who deliver. They are extremely helpful and sell lots of useful bits and bobs. Their advice is second to none also. I get my probiotic powder from here. They will tell you what dose for different animals. The taste of this stuff is wonderful!
I give it to my dogs daily. I initially started it for a dog with repeat urine infections. After reading more about it, I decided to give it regularly because of the health benefits. It's worked a treat for my dog with a perineal hernia! He would frequently have loose bowels (I don't know whether it was due to the liquid paraffin). Anyway, he's now off the paraffin and managing really well on the ACV