Is there any easily-obtained item that can be used to make supercompost?

Pineapples on Brimhaven :) 25 there on the plants and just keep switching worlds until you have enough.

If you have the high enough farming level you can grow watermelons and use those at each different bin to get super compost. Pineapples on brimhaven are good also.

I used to buy pineapples in gnome tree 8-)

Jangerberries are also easy to obtain for supercompost. They're at the ogre island in between Yanille and Castlewars. All you need is a rope for each trip and just pick up the berries (that do have a fast respawn rate) and bank them until you have the desired amount. Closest bank I believe is at Castlewars. If you have a good supply of dueling rings they would make the trips even faster.

You can buy pineapples from Arhein ( the shop guy) in Catherby and then run them to the allotments and drop them in the compost bin ....rinse and repeat until full ...I think he will sell you 25 a day, although I wouldn't swear to that and they are 2 coins each.

yeah, just like Mak_1027 said. you can only get 40 once a day but that's all you need, if you buy everyday =)

I grow bittercaps and put them in for compost too.