Panasonic does not offer an optional rubber eyecup for the FZ18. The one I ordered and installed is actually intended for another Panasonic model (I think the FZ50) and was recommended by someone on this forum.
The part number is VYQ3857 and called an "EVF Cover Cushion". It may be ordered from Panasonic Direct for $8.66 + shipping. The part is currently backordered, with a late January estimated shipping date.
The part is a good fit on the FZ18 and easily installed, with minor modifications. The procedure is as follows:
1. Separate the inner hard plastic insert from the outer rubber cover, Dispose of the hard plastic insert, as it is not needed.
2. Stretch the outer rubber cover over your viewfinder frame, so you can get a "feel" for how it fits. The cover will be a bit tight, but that's a good thing.
3. Remove the rubber cover.
4. Place a couple of drops of super glue on each of the four sides of the camera's viewfinder frame. DO NOT USE EXCESS GLUE OR PUT GLUE ON BOTH THE FRAME AND THE RUBBER COVER. A little goes a long way. I use Loctite Super Glue - Professional (Loctite Part #01-07008).
5. Slide the rubber cover over the viewfinder frame, being careful not to spread glue where the rubber will not cover. Be especially careful on the side where the viewfinder diopter wheel is located. The rubber cover will come close to the wheel, but will not touch it, if the cover is on evenly and straight.
(HINT: The cover is a tight fit and will try to slide around until the glue begins to set (15-25 seconds). Once the proper position is established, hold the rubber cover in place for an additional minute or so. You could probably use the camera 30 minutes later, but I let mine dry overnight, just to be safe.)
Written down, this sounds more complicated than it really is. I have installed two of these units (on my camera and on my daughter's camera) and it's really very simple.
Good luck and good shooting!
--
Bill McClung (a.k.a. 'NC BILL')
Image galleries at
http://www.pbase.com/nc_bill