JP's tended to do marriages. Some resources to find mention of JP's might include: Thomas B. Wilson has published an index and partial abstraction of surviving Ontario Marriage Bonds for the years 1803 - 1834. There are some bonds available beyond 1834. The original bonds are held by the National Archives of Canada in Ottawa and are available on microfilm through Inter-Library Loan and through local Family History Centres of the Church of Latter Day Saints. Wilson’s Ontario Marriage Bonds book is now available with many other invaluable vital records references on CD #204 from Family Tree Maker's Family Archives Series, produced by Broderbund. Unfortunately marriage licences have been lost to us over time as have the records of most Justices of the Peace.
Occasionally the records of a local J.P. will turn up in a private manuscript collection, however, there are no comprehensive collections of J.P. records available. You could also ask Osgoode Hall - they are the experts on Legal History in Ontario. http://www.osgoodesociety.ca