Mike I'm not going to make any specific recommendations here, don't think I can to be honest. What I would say however is that this is an area in which you need to be reasonably cautious. Substances like this are classified as nutritional supplements and as such are completely unregulated. You could in theory find yourself paying a lot of money for rubbish. In other words this is an area where you have to have a lot of trust in the retailer.
The other thing to be aware of is packaging. A lot of nutritional supplements are highly labile and really need to be packaged properly in individual capsule/tablet bubble packs. Proper packaging like this also adds to cost but for good reason. The worst thing to do is buy a large pot of loosley packaged capsules. I would add that this does not necessarily apply to all products but I'm sure that you can appreciate the principle.
Don't have any connection to Boots. I am in fact a vet and prescribe a lot of these supplements.