Of course it's easier to find 3-4 DEs than 4-3 DEs.
Any oversized 4-3 DE or DT can play 3-4 DE...very few can bring a pass rush.
However, it's just as hard to find pass rushing OLBs for the 3-4 as it is to find pass rushing DEs for the 4-3. Even when we had Ware tearing it up at OLB, we never found a great one to pair with him...Spencer was our best, and he was nothing special. When Ware played DE, we couldn't find anyone for the other side.
Most teams never have a 12-15 sack DE or OLB; it's extremely rare to pair a guy like Ware with a guy like Miller. In the absence of a stud DE, you can still help your entire line with a stud DT like Donald or Cox. Best pass rusher available at any position is the way to go, not just DE or bust.