In regard to where to buy quality feed oats, I would suggest contacting local feed mills, feed stores and often times coops have clean whole oats. When inquiring be sure to ask for "cleaned" oats otherwise they have a lot of chaff. The other distinction is to ask for heavy oats as those are predominantly the feed quality oats.
I know it might be a ways away, but Venell Feed south of Corvallis handles whole oats and they are a really good company to work with.
There are a couple of other feed mills like Union Mills in Mulino. There is also Old Mill Feed and Garden in Dallas. But they might have just bagged oats.
In regard to feeding a horse with HYPP...research has found that grass hay and whole oats are a great combination as that combination is low in Potassium. The most important thing when feeding a HYPP horse is to feed low potassium feeds. Feeding 3-4 times per day also helps these horses by reducing/avoiding nutrient spikes. It is recommended a minimum of 3 times per day feedings. It is also recommended to not exercise the horse within 2-5 hours post feeding to help reduce an episode.
Some feeds to highly avoid are: sweet feed due to molasses being high in potassium, straight alfalfa hay, and soybean meal. Alfalfa-grass hay mixes are ok to feed but not as safe as straight grass hay. Beet pulp in moderation is also safe.
I have more in-depth information regarding requirements and diet formulation, but I'm not sure how in-depth of information you would like. The amount of hay and oats to feed is dependent upon a multitude of factors.
If you would like more detailed information please feel free to contact me at the phone or email address below.
Have a great day.