Try Kelly blue book (KBB.com) varies as to package, location,etc.
I too have an E150 from 1995 and the fuse box is hidden behind a wooden panel on the driver side wall just ahead of the first captains chair.
In order to get to the engine, you have to first undo the clips holding the engine cover on. In mine, that meant lifting the floor mats and pulling back the carpeting a bit (Glaval Primetime Conversion). There are also two bolts holding it the floor which also have to be removed. Both front seats also have to be set all the way back on their tracks in order to maneuver the cover out of the vehicle. Once the cover is off, you remove the plug caps and wires (you should also replace these when changing the plugs) and remove the plugs with a standard plug socket. Be sure to properly gap the new plugs before replacing them.