Keavin -
You can buy a RC4136N at Mouser Electronics:
http://www.mouser.com/Search/Refine.aspx?Keyword=rc4136 It's only $1.30 (probably $7 shipping, so buy a bunch).
This device is a really generic op-amp.
You actually quoted the RC5136DB which is a slighly different chip. It looks the same, but is a surface-mount part where the legs don't protrude through the PC board. These parts are designed for robotic assembly. If the chip you're looking to replace is soldered directly to the PC board, then it's possible that this is what you want, but if it's in a socket, then you almost certainly want the 4136N. If it's directly soldered, you'll need somebody with specialized experience to replace it.
If this is in Alembic #12, then you want the 4136N to replace the socketed amp.
Keith did a good job sleuthing. I think he meant SOIC layout, which is actually a newer package than the DIP chip that's in your bass, but was obsoleted first. The guts of this sort of chip would have just been buried inside a bigger chip in the modern world, but the old-school discrete ICs (e.g., chips that have just one function in them) often outlive the newer parts.
Take a close-up picture of the chip you want to replace and we can determine for sure.
David Fung