marco currently only one has them in stock. Search for 6821

I bought several 6821PIA's from Mouser Electronics about a year ago. Maybe they still have em'. They will be considerably cheaper than Marco.

A 6821 PIA (or a 6820) are the same part across plenty of pins and other devices. No game-specific or proprietary code on them.

I get mine from Todd @ Big Daddy
http://www.bigdaddy-enterprises.com

If you do decide to replace the chip be sure to add a socket. I use machine pin sockets, it is only a few extra dollars and really is considered the correct way to make an IC repair.

On something like a PIA, while machine pin sockets will work nicely, I like to use the socket strips. They have machined pins, but lack the 'internal framework' of a socket, so you can solder the leads of the socket to both the top and the bottom traces of the board. That's especially important when you're learning, but even applies to a seasoned technician - the traces on these boards are kinda fragile. Don't let that discourage you - just be careful and double check your work!

http://www.greatplainselectronics.com/products.asp?cat=44

Scroll toward the bottom of the page. The strips that John is referencing are out of stock at GPE but you will get the idea and can go find them at another vendor.

I buy my 6821's at Jameco:

http://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?freeText=43596&langId=-1&storeId=10001&productId=43596&search_type=jamecoall&catalogId=10001&ddkey=http:StoreCatalogDrillDownView

Their stock number for it is 43596 and just $3.95 each.

Todd has them in stock

http://www.bigdaddy-enterprises.com/electronics/e_sockets.htm#sockets-sip