How do I achieve a lower gear on a Norco Bigfoot with a 9 speed cassette?
Looking at the spec, your bike has a double front chainring.  Are you shifting into the smallest front ring when climbing?  If not, then do so.   

You can possibly buy a smaller front chainring too, that will leave the higher gears alone for faster/road usage, like downhills.



The single cheapest solution is to pace yourself by slowing down, harden up your muscles and climb at a rate that you can sustain.   This is the most economic option.

Is the grade steep enough that you physically can't ride it?  Consider taking an alternate route, or changing a straight climb into a series of switchbacks.



A 9 speed cassette is still available to buy - simply look for one with the biggest big cog and see how it goes.  You will need a new chain too.  Converting your existing cassette won't work if its a one-piece unit, or if the smallest cog is the lockring.

Example http://www.wiggle.com/shimano-alivio-9-speed-cassette/ is a 11-34 tooth for quite cheap.