Smallbear sells a ua308, same as LM308, except made by Fairchild for $5.50. Also try looking for LM108. Same as LM308, only difference is 308 is rated for operation from 0 to +70C, 108 from -55C to +125C. I have a few NOS LM108s, put one in my Expandora / RAT clone, sounds great. There are probably a ton of other single opamps that would sound good in the RAT. Install a socket and try a few. Also, don't worry if the 108 or 308 comes in a metal can, the leads can be easily formed to fit a DIP footprint. I am very wary of buying semiconductors on Amazon or eBay; there are too many vendors that do not understand ESD and might be selling wounded parts.