You don't have to buy specific bags, just use an old compost bag or similar. I use large buckets which hubby got from work. I've had quite good yields, and one benefit of growing in bags rather than the ground is that there's much less damage to the potatoes. I've had lots of slug damage from those grown on the allotment, but no damage at all from those grown in containers.
Ho Bob, having not read al the replies, I may well be repeating other comments and I have mentioned this in other posts but in case you have missed them, I recommend the following planter
I also like using polypots which I find a lot easier than digging in the ground