Denim is going to be a little tricker but you should still have some good options at Kohls or at JCPenney. I would suggest pull-up denim style pants (my own mother loves these). One of my favorite places to get denim inexpensively is your local thrift store. This will vary by state. For example, in Nebraska, the Goodwills sort their denim by size and you can walk in on Monday mornings and they have a new "color" at 99 cents -- browse quickly through the denim for elastic waist or go through the maternity denim section and you're good to go to quickly see whether they'll have anything that will help you or not.
In contrast, Goodwills in Texas don't sell anything under $6.99 or so and they offer no discounts on any day of the week for clothing. Also, here they are not sorted.
So, depending on your state you *might* want to try secondhand.
I still think -- even with denim -- you might want to call Lands' End customer service and get those "made for you" pants. The company would have them new at a relatively reasonable cost and would fit her perfectly as well as having a wonderful guarantee.
Sounds like you have a good bargain hunter on your hands -- you should be good to go! The thrill is in the hunt, right?!
I would suggest Kohls or JCPenney (or even Sears) before Gap or Old Navy. You might even try the petite section of Dillards or Macy's as they have great clearance sales (really! Macy's had a t-shirt for $3 the other day!). Just gotta be willing to go back and "hunt" which it sounds like you are!! My kind of shopper!!