[doganjo]

Beware though about dumpy sacks, the plastic woven ones are degradable and crumble to pieces in two years.

But that's fine, you can get more, the builders don't seem to re-use them any more and they are left at the local tips/recycling centres.

[valr]

I got brilliant bags in poundland - 2 for a pound - green garden refuse sacks. They "pop up" into a cylinder. I put drainage holes in and am growing potatoes in them. Have used old compost sacks in the past but the tops flop about and cover the tatties half the time - these are much better. Bargain!
They also do proper "potato grow bags" which are reasonably good and re-usable..
It is worth checking out thes cheapie shops like poundland - most do really cheap fleece and netting.