There are some strategies required in each of these recommendations. Goodwill moves higher quality items fairly quickly, so you have to stop by every week or two for a month or two, maybe, to catch something that's good quality. You might hit it the first visit, or it might take you several. Then I'd have it dry cleaned. But examine it thoroughly, and have it hemmed or taken in a little as needed. Some re-sale shops have low prices but are more selective with their inventory, so you may have a better chance of finding something higher quality.
Department stores have great sales, but you have to catch the old season. Summer suits will go on sale after July 4, and you can often get them half price. Likewise, winter suits go on sale after New Years, but on even deeper discounts in February.
I like J. Banks, but I'd buy only on sale, and be careful about the two-fer or three-fer sales. You usually end up paying approximately the cost of that number of suits on sale; that is, the prices are set to you don't get as good a deal as you'd expect.
Goodwill is the best deal, but you'll have to work at it, probably in repeated visits. Don't get anything that is conspicuously outdated. Check out all zippers, buttons, and make sure the lining is in good repair. Sometimes a sports jacket is a better workaround than a suit, unless you need it for weddings or funerals; today they even work for those, since people show up in chinos and sports shirts these days -- you'll look dressy in comparison.