Where can you get an original owners manual for a giant boulder bike - or one for 'Giant' bikes in general?

It's unlikely that there ever was one.

Luckily, you don't need one. A generic bicycle maintenance book will tell you all you need to know and then some.

If there's anything missing, most component makers, (Shimano, Rockshox, SRAM etc) have good tech support documentation on their websites.