Try going into local pharmacy and asking what they recommend for humans, most of what is safe for us is safe for horses, and if you look up the active ingredients in mg and compare to horse stuff you should get an idea of value.
i got my non adhesive dressings from Boots, the identical stuff at the vets was twice the price.
Apple Cider Vinegar - but must be at unpasturised and at least 5% acidity. The live natural stuff with the cobwebby bits in is the best. I use the Hilton Herbs one. NAF do one but you have to feed almost three times as much as the Hilton Herbs one so although more expensive to buy it works out cheaper!
I had my old lad on Blue chip Dynamic....as it is in pellet form it is easy to feed alone without having to mix into a feed...handy when your horse lives on fresh air...and if I ran out... I noticed I could hear him clicking !!
Hi I bought some linseed oil to try and improve my boys coat. Imagine my surprise when the navicular suffering stiff old man started moving the best he had for years! I have no idea whether there is any scientific basis for this but linseed oil certainly worked for him (I give a couple of tablespoons a day for a 16.3 cob so cheap as chips too )