Equivalent of S27c512-20A PLCC for project
I searched eBay for 27C256 and 27C512 and found loads at low prices. The way I read the article, you only need a 256, the 512 doesn't seem to get you additional benefit. 

If you can find a datasheet for the S27c512-20A, you can verify which alternatives have fast enough timing.  

It's likely that the '-20A' means 200ns Max access time/min cycle time, so parts with a faster access time (suffix less than 20 should work).

You will of course need a programmer for these parts... do you have access to one? And software to run it?  If not, this thread may be helpful: http://www.eevblog.com/forum/microcontrollers/help-in-copying-a-set-of-eprom-%2827c256%29-and-programable-memory-9306/15/

[Edit]

I've recently noted the availability of relatively inexpensive programmers on eBay and Amazon, including ones that can be driven from USB. So that would be a good place to start hunting, if a programmer is needed.