According to WorldCat, you have a book titled "Mercer County, Pennsylvania Will Book Index 1804-1900". Is it possible for you to lookup a name
In case you run across another Lebbeus Simmons (different spelling is deliberate), I'm also looking for his death (He is Father of the other one). He was in Mercer Co, PA in 1870, so sometime after that (he was about 75 then).
I don't find a Lebius. The only Simmons wills in this index are:
and William (1900)
Many people did not make wills. I would also check estate/probate indexes--FamilySearch.org has digitized the ones for Mercer Co. (and all kinds of probate records for other counties as well). These records are not indexed, but you can page through them just as you would the microfilm or the actual book.
I hope this helps!