The J. C. Penney line of Stafford suits can be bought as separates. Fairly conservative styles. Penney's seems to have frequent sales in the men's dept.
Edit: I forgot that Jos. A. Bank also sells several lines of separates (more $ than the Penney's stuff).
Macy's too. I have a Calvin Klein suit that I bought there. I needed it right away and didn't have time to get a "real" suit tailored (although I still got it tailored later on). I was there last weekend and saw a bunch on sale.
If the pants and jacket are made with cloth from different dye lots, the colour can be off a bit, causing a suprisingly bad look to your 'suit'. Hopefully if you buy all the pieces at the same time you might have a better chance of getting pieces all from the same dye lot, but there is no guarantee. Coming back a year later to buy an extra pair of pants ... good luck.
Of course, if you can buy in person rather than over the internet, you stand a better chance, too.