I don't think you'll find brewers yeast in any supermarket...and the health food store stuff is sold as a supplement; a vitamin...it's gotta be a brew shop. I could be wrong, but I rarely am.
In general I use liquid yeast - but I keep some Safeale around in case I decide to brew up something off the cuff.
Safeale is a dry yeast with good quality results - I think the same company makes a lager yeast if you are interested.
The stuff that comes in kits is usually good for providing some variety in your wastebasket - you never know how old it is or what conditions it has been kept in and is not generally of high quality to begin with.
I have never seen brewers yeast in suppermarkets before. I've been using the Wyeast for a long time now and no problems, starts to ferment in 2-3 hours after pitching.
Just alow time for the yeast to cultivate before pitching.