Many churches (most graveyards are close to one) + other graveyards are easily accessible by public transport - don't forget the Island is only 30 miles by 12 wide so distances are not great. Some churches (old Ballaugh, Old Marown + Old Lonan) require a good walk. Taxis are available (I assume you would not consider visiting in TT fortnight) - you can hire the help of blue badge guides - your best bet here is to go via the Government Tourist Board - look at www.gov.im - who can also supply all other info including tourist passes for all public transport which is easily accessible for all.
A visit to the library of either the Manx Museum (in Douglas) or the IoMFHSoc library in Peel would equip you with plans of the various yards you intend to visit otherwise finding a specific grave might be very time consuming.