The 4.5V battery is still available. The battery type was "531", sometimes known as "A19PX" and was an alkaline, not mercury. Some of the 900 series roll film cameras required a small 1.2V mercury battery for their exposure meters. Good reference site for Polaroid cameras is the Land List, here is the page on batteries :
http://www.rwhirled.com/landlist/landbatt.htm
Googling for "531 battery" and you get lots and lots of sales hits. However the battery itself isn't particularly cheap. Last one I bought was through Freestyle Photo but only because I was ordering other stuff. You can get them cheaper.
Or you can do the modification and use three alkaline 1.5V batteries in series. AAA and N cells will work. You can find plastic N cell holders through distributors like Mouser (
www.mouser.com) made by Keystone. Or you can solder the batteries directly. But work quickly and cleanly or you cook the electrolyte and just have a dead battery with a wire soldered to it. You can buy 9V battery clips and hack them apart to get the mating pieces you need. The black wire is the low potential (-) and the white wire will be the high potential (+).