There are a few on Alibaba https://www.alibaba.com/trade/search?fsb=y&IndexArea=product_en&CatId=&SearchText=planter+gauge+wheels  
I contacted one of the suppliers just for fun and they told me there was someone in South Dakota that ordered a boat load and was reselling them.  They mentioned could be had for $30/piece directly from them (supplier in china).
This won't help anyone now since it'd take a couple months to get here but just for future reference.