Thrift shops--should be able to find fleece jackets for insulation and very possibly hats and gloves
Military surplus stores
Big box stores such as Target, KMart, Costco, Walmart (in the athletic department, not the outdoors dept., for wicking layers and nylon shorts/track pants; may also have fleece although it's the wrong time of year) If you have a Big 5 Sporting Goods in your area, they often have various outdoor clothing on sale cheap.
For thrift/cheap fleece, look at the light through it before buying to make sure there are no holes/thin spots. I personally would not buy the pricey fleece found at outdoor stores when you can get it for a fraction of the cost from thrift stores.
I see no reason to ditch the Dri-Ducks. Just test them in the shower (might have to seal the placket seam). The pants are usually the first to go because they are more stressed in seat and inseam. It wouldn't hurt to take one of those emergency plastic ponchos as backup, but if you don't go crawling through brush, the Dri-Ducks should hold up just fine.
Costco (if you have access to a membership) has good merino wool socks.
Might be able to find an inexpensive nylon windbreaker in either thrift or big box stores to use as a windshirt, although you may have to cut out the cotton flannel lining. Put a DWR treatment on it to make it more windproof.
There have been several clothing ads the last few days on the BPL buy and sell forum, although I haven't looked to see what they are.