In [106]: def func1(mystr,sub,k):
    inds = (i for i,c in enumerate(reversed(mystr)) if c==sub)
    return len(mystr)-tuple(islice(inds,k-1,k))[-1]-1
   .....:

In [107]: def func2(mystr,sub,k):
    return [i for i,c in enumerate(mystr) if c==sub][-k]
   .....:

In [108]: strs="abcdabababcebc"*10000

In [109]: %timeit func1(strs,'b',200)
10000 loops, best of 3: 129 us per loop

In [110]: %timeit func2(strs,'b',200)
10 loops, best of 3: 28.4 ms per loop