Well, I am not plus size but some of my most favorite suits come from Marshall's. My favorite brand that I get their is Tahari because it fits me best. I like the style of Anne Klein suits but their pants are cut large, so if I get a jacket that fits, the pants are too big. They work for my friend though. Usually at Marshalls, the suits are about $99.
I do have one suit from Ann Taylor but I don't like it as much. It seems like the slacks pick up every bit of dust in my office and my blouse collar NEVER will go over the suit collar/lapel properly. These are things you need to look at when buying a suit.
Also, be a little careful at Penney's. While they have a big, affordable suit section, I find tha many of their suits aren't "classic". You *might* find one. These suits will be fine but there is usually always some weird, stylish type thing on that such as an offset trim on the collar, too-decorative of a button, or something that takes the suit away from being a classic. These are fine, IMO, once you have a job.
I also don't have a problem with black. Just make sure that you get a blouse underneath that isn't dull. The beauty is you can go with anything because only a bit of it is showing. I get the most compliments on my black suit when I wear it with a white blouse that has a "hot pink" plaid throughout. Got that a Target (Isaac Mizrahi). One other thing to pay attention to is the pants leg on the suit. I have found that Dress Barn suits have a very straight or "peg leg" opening for a lot of the pants. When I put the suit on, it screams 80s to me. You will also want to stay away from a pants leg opening that is extremely wide (for an interview suit). Find one that has a nice gentle flair that will glide over your shows but not be too tight or too billowy. Ann Taylor suits, Tahari, and Ann Klein all fit this description.