Sliding windows implementation (Exponentiation by squaring)
Serious cryptography software today does not use sliding windows for exponentiation by secrets precisely because it leaks the secrets through array indexing.  You might find it older versions of OpenSSL, libgcrypt, etc.  In modern software, your best bet is probably libgmp (mpz_powm, not mpz_powm_sec) or other generic bignum libraries optimized for speed, not for security.