Greetings Balthazar and Welcome!!
In Northshield, we do have Equestrian over the summer, we are most active on the NS Equestrian Facebook group if you use that tool.
https://www.facebook.com/search/top/?q=northshield%20equestrians The next upcoming equestrian event is Bardic Blades in Elgin ND 8/3-8/6, and Poorman's Pennsic in Wisconsin Rapids, WI 8/11 - 8/13. This year the wonderful Shire of Falcon's Keep (Wisconsin Rapids, WI) is also hosting a monthly equestrian practice. I have to check the dates on when those are scheduled, I think it is 3rd weekend of each month except August running thru October but will confirm.
We could definitely use a Fall event, but be aware our Kingdom is large and our Equestrians are scattered. So depending on where you were thinking about holding it and when, would determine attendance.
Eowyth has sent you some links below, here is the link to the Society equestrian home page.
http://www.sca.org/officers/equestrian/index.html Northshield is currently using Society rules as our Kingdom rules, with no additions. State rules apply for what specific travel papers you need. In terms of participation, for a practice, unauthorized riders can attend, train, and when ready authorize. For an event not classified as a practice, you must be authorized to participate. If you don't have your own horse, we'll attempt to assess your skills and hook you up with a horse owner who might have a mount that is suitable.
Stall fees and site fees depend on the facility in my opinion. Primitive site with grass arena, panel stall pens I would expect to be much much less than a super large, well groomed arena with high end stalls, and everything in between. Feel free to float a proposal, we can get you some feedback for sure. And we hope to see you at events!
YIS
Alissende de Montfaucon