Congratulations! I'm a mom of twins, too, and know all too well what you've been through just getting to this point.
I carried my twins to 38 weeks, so I was pretty darn big by the end. Fortunately, it was summer time and I wore a lot of maternity capri pants and sleeveless cotton dresses from Target. I outgrew my dressy work clothes at around 6 months. In addition to Target and JCPenney, I would suggest Kohl's (if you have them where you live). They had good prices and it wasn't just tight-fitting stuff.
Also, see if there is a Parents of Multiples group in your area. I joined a group after my twins were born and we hold Mom-to-Mom sales each spring and fall. I wish I had known about the group before they were born because the other moms sell their maternity clothes at great prices.