When a certain kind of crop runs out, just select Greg on your friend’s tab and buy everything in his roadside shop. You can find some tools in his shop that are very cheap. But what if Greg doesn’t have an item that you want? Just visit other people’s farm, survey their newspapers, and buy the items you need in volumes.
For levels 7 and up, if you’re missing some kind of crops, just look at the newspaper and see what’s on sale. You can also go to your friend’s farm and look for their roadside shop to see what they’re selling.