This achievement can be completed very early in the game. Most pieces of clothing are more expensive than $1000.
Easiest way to get $1000 early in the game:
-2-3 side missions will net you $1000 easily
Or
-Hunt and skin animals. Bears and wolves being the easiest early on. It's worth avoiding any wolverines unless you're feeling agile to run and gun the little bastards
Or
-You can sell maps and magazines that you pick up, usually $150 - $200 a piece. Once you pick them up you already have the locations marked on the map and they can be sold without losing the map items