I actually take my stuff to men's wearhouse and have them help me pick it out.
There is such a thing as over dressing for an interview. I like to find out what they wear day to day and step up one or two notches.
+1..
I've taken stuff into Mens Warehouse as well. Tell them what you need it for, show them what you have and they will lay out a few of the missing pieces for you as well as offer up maybe some other options.