When I found out I needed to interview for a fellowship on 3 week's notice with "professional business attire" specified for the interview and one of the weekend's dinners (and didn't really own professional business attire) I freaked.
Luckily spring break was during the 3 week span so I went scrambling around with my mom to find interview attire.
I wanted to go to Express, but we started at Macy's - their junior department was pitiful (cheap quality "suits" that looked horribly unprofessional and tacky) but their regular department had a wonderful selection of high quality designer suits - and they happened to be on sale! Between the sale and a coupon we cut from the newspaper, I paid about $138 each for a very professional looking lined black pinstriped skirt suit (both pieces included) and a lined grey pants suit with a pale blue sweater/shell that matched the faint pinstripe in the material (all 3 pieces included). Just a blazer at Express was about $128/138 and the quality was far inferior. Both suits retailed at $300+ but department stores like Macy's always have sales.
I tried on about 50 different suits at Macy's and found that Tahari fit me very well to the point where I didn't need alterations. Other suits looked silly on me, but knowing that there was a line out there that fit made it easy for me - I know what to look for in the future.
I had a lot more trouble looking for dress shirts that fit - especially a basic white one (most stores had shirts with detailing or stripes which would have looked funny with a pinstripe suit). Short on time, I finally bought 2 dress shirts (button down) from the GAP - white and blue (They were on sale for $24.99 and $14.99 respectively). I also bought a lime green shirt from Burlington Coat Factory on sale for $12.99 which I ended up wearing to my interview so I would stand out.
I got the fellowship so it must have worked
Depending on how conservative you want to dress, I'd say department stores can be wonderful! Head straight to the "real" suit section and shop the sales - My suit was anything but frumpy (I'm 5'4" and a size 2) and it hit at the knee and the blazer was fitted. They had women's, petites, etc so I'm sure there's something for everyone. The quality is better than most of the NY&Co/Express stores and if you time it right you can get a much better price too.
I googled and found suits similar to mine but I'm having trouble linking - go to the general page here:
http://womens-apparel.smartbargains.com/ and look for tahari
edit: I also found a beautiful tweed skirt suit (young looking) at Nordstrom Rack - the blazer was marked down to $59 and the skirt down to $19.99 - So far I've worn the blazer with tweed shoes and a black tulip skirt from express but it also looks nice (and more conservative) together