Feerrott Fern Leaves Collection.
You can buy all the leaves from the broker for about 25g, although the red leaf may cost up to 20g itself. I was fortunate to be given that leaf from a guildmate. Then head to NQ to turn in the quest - viola!
It's not the cheapest, however, if you have some cash to burn and your current packs are filling up with all those reversable writs, this bag is in the bag. :)
...I can remember in the good old days when a 10-slot bag was a huge item. :)
I also carry a 24 slot box. Bad news is that it adds 100 in weight - I have a 47 STR. But so far I have filled up everything and still had a few pounds left over. Only difficult time is when I get hit with a 15 minute poison/disease trap that reduces my STR to 12. Completely overburdened really means completely...I basically crawl for 15 minutes. GOod thing my pet can reach far. :)