There are three copies on ebay, two of them not so badly priced right now...

Gamestop: http://www.gamestop.com/search.asp?s....x=18&find.y=9

EBGames: http://www.ebgames.com/ebx/categories/products/deptpage.asp?wherefrom=search&searchword=space%20i nvaders&web_dept=Nintendo%2064

Amazon: http://www.amazon.com/exec/obidos/tg...e&s=videogames

Half.com: http://half.ebay.com/cat/buy/prod.cg...1794&meta_id=4