The area that we live in is pretty much all farmland. That being said you would think John Deere supplies would be easier to find. No such luck. We live in the middle of no where. Time to think outside the box. We made a refueling station for the kids. An old water jug filled with gingerale for "gas".
I find that store bought cakes are way too sweet and have too much icing EVERY SINGLE TIME. Not that I've ever bought one. I make my boys their cakes or cupcakes. However, I'm just speaking from experience of other cakes I've eaten elsewhere. Takes me less time to make it than it would be to buy it. The animal figurines from the Dollar Store add a little farm detail and the kids were happy enough with them.
Hay Bales aka Rice Krispy Squares. Sorry for the bad picture, I was multi-tasking at the time. Just make Rice Krispy Squares how you normally would. Cut them into "bales" and pile high!
I hope this gives you some ideas for a farm party! We had plenty more things...like bobbing for apples and ping pong races. The skies the limit. Just have fun.