Sometimes throughout your adventures from playing the game through "exploring." You can sometimes find the stuff just by caring for and entering your horses into competitions.

You can also get Whistles, Oats, Alfalfa, Carrots, Apples, Clover along with Molasses. You can combine Carrots, Apples and Clover with Oats and Alfalfa to create balled treats, using Molasses you can create sweet treats. If you do get enough, this is found under Account --> Craft Room --> Cooking. Oats/Alfalfa/Molasses is considered Item 2 while the others is Item 1.

Lately though, even I have been struggling finding stuff that isn't Lassos and or Whistles. Occassionally, you can find folks selling off these items under Market --> Marketplace -->Newest Trades but that sometimes can get steep in pricing. You could also do ISO horse catching items and have people private trade the stuff to you as well.