Haven't read the other thread, but if its particularly joint supplement you are after, I buy Cosequin which is very effective, from the States through Entirely Pets. They offer discounts often, fast shipping and extremely good prices. Even with shipping costs and import duties, works out virtually half price from the UK and cheaper and more effective to feed than most other supplements.
My old mare has multiple problems and it visibily helps her more over Cortaflex and several other leading brands.
For general supplements, I like Equimins and Feedmark.