Shimano makes a 9spd cassette with a 12-36 range. Its is the CS-HG400. You will loose lose some top end range but gain on the low end.
It is a heavy cassette, but it shifts well and is durable. I used one for a season on a 29er with a 32t and 30t chainring, depending on the trail.
I would recommend a new chain when you do the conversion. You could even use a 10 speed chain. It will shift fine and stay on the narrow wide ring better since it is narrower. Chain length is key since there are no 9spd derailluers with a clutch. Basically you want the chain as short as possible to keep enough tension on the derailluer cage spring in the smaller cogs, without ripping the derailluer apart if you bottom out in the large cog.