I've had the same dilemna.
The problem with cellulose, i.e. old shredded newspaper
treated with nasty fire retardant chemicals, is that it settles. It's great for attics as blow-in insulation, but in walls it will lose it's loft and thus most of its insulating power.
You can buy wool insulation in batts just like fiberglass but it's very pricey. I once used old packing material, mainly bubble wrap. It was a very temporary solution but it definitely did the job
. I suppose if being to code is not a concern you could use that and old styrofoam. You need to mak sure the walls are sealed, though, as any air flow (it seems to me) will affect it even more than with fiberglass.