@frostycab
I gave up buying in the UK either from shops or online a few years ago. I found great inconsistencies, which I suspect are down to how they are stored by the supplier. None of the brands show any trackable supplier information. I usually pay £8 - £10 a large bottle online.
As to the hit/buzz/rush. I have a suspicion that the body adapts to use of a particular brand and so the hit is not so good after a while. You change to a different brand and back comes the hit. I don't really know. I remember dancing and giggling with poppers when I was young but I don't get that rush nowadays!
As to the suppliers - I suspect we are not supposed to give details. They advertise online though, there is one I use in Germany and one in France that have been around forever. If you look on their sites you will see they give proper information as to what they are supplying. If the site you are looking at don't do that, go look at another site.