4164s are very common chips of the era and often pop up on ebay.
The problem you will face is you have no way to know which chip is the faulty one (or more than one) on the C64. These are 64 Kbit chips but they are 1 bit wide, by that I mean that at each of the 65536 memory address locations the chip only holds 1 bit, so the C64 (and most machines of the era) use 8 of them, each chip providing 1 bit for every byte. The address lines are common so every chip reads the same address at the same time as all the others, and their outputs are carried separately on the Data bus. So, any chip could be the culprit, as every chip has the ability to bugger up every byte in the RAM stack. Finding out which chip is dead without a scope is going to be almost impossible. You might get lucky if the output pin is stuck high or low, but memory chips rarely die in that way, usually the output is flapping about but not being useful, so a logic probe is useless.
The second problem is that it is very hard to get a chip off the board without a proper desolder station, solder pumps and solder braid are next to useless I have found for clearing solder from around chip legs. You will probably do more damage to the board with multiple attempts at getting the solder out. The chip will probably survive but its not certain.
Not the news you were hoping for I imagine, also piggy backing RAM chips on the old ones has about 50/50 chance of showing anything useful.
I would happily fix it up for you, but I guess the postage would be a bit steep for such a common machine.
