I get mine from horsehealth.co.uk. Pretty reasonable on the price, and orders over £100 get free delivery by courier. The were also extreemly quick at delivering. I ordered mine on the Saturday night, and had it first thing tuesday morning.
Had a quick look round when I bought mine and they seemed to be all selling at similar prices only variation was P+P. Got mine from supplement solutions in end. Good service, reasonable P+P and free pack of horse treats thrown in!! I did also contact horse first to start with re local stockist but there weren't any near me.