There are many old and interesting cemeteries in Philly, of course, since it is such an old city. However, it is hard to remember what type of fencing each cemetery has (!). I think most of the ones in town now have sturdier stone or brick fencing, for the most part.
All three of the above are very worthwhile anyway, if you are interested in old gravestones and history My favorite is St. Peter's. There is a very interesting cell phone tour you can do as you walk around. Info can be found on signs in the cemetery.
Just a guess, but if you were really serious about old cemeteries surrounded by old iron fences, you might have to research some of the ones further out, such as in Germantown. Exploring these areas is going to be challenging on your own, even with a car, I think.
Unfortunately, old iron fences have been a target for thieves for years, one reason why they are harder to find. Iron fences were also used for housing fencing, but not much of it left now.