Thanks, everyone.
I was able to find the solution by putting this in
C2: =VLOOKUP(A2,$B$2:$B$400,1,FALSE)
C3: =VLOOKUP(A3,$B$2:$B$400,1,FALSE)

Column 1 had all the account numbers from one source. (last row: 348)
Column 2 had all the acount numbers from a different source. (last row 311)