For the sake of future viewers of this thread, here is the answer: a Laban #5 nib will fit a Laban Celebration fountain pen. The bigger question then becomes "where can you buy one?"
If you go to ebay.co.uk and search that site for "Laban fountain pen nib" you will find a seller, Andreas, of a1-cufflinks, from Nicosia, Cypress. Well, at least as of September, 2013 he was on the UK eBay site.
No big surprise, but the Laban nibs are made by Bock in Germany. Andreas sells them for about USD20.00 (actually11.95 UK Pounds) including shipping. Add another USD 3.12 if you want faster shipping.
I bought two, a M and a B, but he also offers F. He also carries Laban #3 and #4 nibs. You even can get a #3 in 14k gold. He has a 99.6% positive score on over 5200 Ebay feedbacks.
For the sake of full disclosure, I do not work for him nor have I ever bought anything from him until today. I thought that other Laban owners who need new nibs may want this information, as it took me a lot of digging to find someone who could tell me which nib I needed and who sells the nibs.
I will report back to this thread when I get the nibs and get one installed and inked.
Edited by AV8R210, 25 September 2013 - 15:53.