To get more supply the easiest options are :
- Take Nature magic and Growth scales, it gives additional supplies
- Choose a good Castle/Fortress, that has a high Supply value (Fort.Cities are best, but very expensive..)
- Build supply-creating items - most are Nature-based - and give them to your commanders
- Use rather a few good troops that many crap ones, they eat less
(playing Ulm for example I rarely have supply problems, but much more with Machaka ..)