Well, you are not going to get the quality that one needs from the chains or the drugstores. I recommend always going to an independent health food shop, and finding a place that knows their business. ~~points at self~~ I work at such a place, and the owner demands that we all train to become certified nutritionists, with working knowledge of the supplements we sell, which ones are the best, and why, ect.
Take evening primrose oil, just as an example. You see lots of places selling "high strength" and slap 1000 mgs on it to make you think that you are getting something good. Milligram strength is really irrelevant when it comes to essential fatty acids, as it isn't the mg dosage that is the important part, it's the GLA percentage that is vital. GLA (gamma-linoleic-acid) is the active ingredient in evening primrose oil - it's what causes results. There are loads of places which sell evening primrose oil with a high milligram dosage but very little GLA percentage, like 1% or 2%. Rubbish. Some of them don't even tell you what the GLA% is! Stay away from them! You need at least a 10% GLA for it to work and be beneficial. Sort of like wine. If I tried to sell you some wine with only 1% alcohol, you would tell me I was crazy; you know that that wine has no strength, it's very little more than grape juice. Alcohol is the "active ingredient". Same with the evening primrose oil.
Anyway, I won't go on....people tend to tell me to shut up (well, except when I'm in the shop, and they've come in for advice, that is) when I start in. But, you'll feel more confident (and know that you are - usually - getting a better product) if you go to an independent retailer (and, preferably, one who doesn't sell their own label, so they can give you unbiased info, like we do at Healthy Living).
Don't know where you are, HantsMamma, but if you are in the East Midlands, we have the three shops: Leicester, Wigston, and the one I work at, Loughborough.
Hope I was more of a help than an annoyance...