Anyone know where to find such a thing?
What are you looking for?
How far back do you want to go?
The USEF rulebook archive page goes back to 2004
www.usef.org/_IFrames/RuleBook/archivedRulebooks.aspx
Or just go to www.usef.org
select Rules and Regulations
select Rule Book
select Archived Rule Books
select the year you want
select XVII Eventing, or EV, depending on which year it is .