Probably part US-1085 on this chart:
http://www.stengelbros.com/trailer-suspension-parts/trailer-leaf-springs/I replaced mine with the US-1091 and it rides better than ever.
Take the chart out with you and measure. Only a couple hours right?
The real trouble will be bringing all the right tools next time out to change it yourself. Take tools on the first trip and verify you have everything. Make a list.
Good time to upgrade to a wet bolt kit with everything torn down. And of course I recommend replacing all springs at the same time.