I'm going to third the suggestion for thrift stores - if you have reputable ones (or even a good Goodwill) then that's often the way to go for "in-the-middle-of-losing" clothes. I will say that buying new clothes always feels great and can be a great motivational booster. You don't need to buy a whole new wardrobe, but new jeans and staples are, I think, important, especially for on-the-job if you don't want to look sloppy. I will say that I've purchased some nicer, more expensive clothes in the size I am now with the intention of taking them in/altering them when I reach goal weight, because some pieces simply look awesome and it'll be worth it to keep them around, even when I'm smaller. There are a lot of options, but I totally understand the fear of suddenly waking up to see the "old you" in a few months or years from now!!! Happens to me at least once a week, or even if I eat a cookie or something, lol.
On the flip side...I've kept a pair of my old size 20s for comparison purposes when I get to my goal weight, just to see the difference. That's motivational too - sometimes I put them on just for fun to see the change!
Female, 30 years old, 5'4 1/2" tall
Starting weight 1/4/11 = 215.2 lbs.
Weight 5/19/14 = 179.6 (net: -35.6)
Current mini-goal: 175 lbs.
Lowest weight: 156.7 lbs.
150.2 lbs. <--- Official "Healthy BMI" weight
Estimated final goal: 130-140 lbs.
"You don't have to change your life today. You only need to change your day today."