I have always been amazed that the surface-mount 68040 versions have been so much harder to find for sale that the socketed kind. Well, I just found somebody who has many of them. They are an ebay seller who I am not affiliated with. They offer NOS XC68040FE33M chips in a few different auctions. These can replace the LC 040s on the PowerBook 5xx CPU daughter cards. Won't provide the larger LCD of a 550c, but probably the next best thing.

This auction is a dutch auction with single chips at $50ea:

http://cgi.ebay.com/Motorola-XC68040FE33M-CPU-QTY1-Upgrade-XC68LC040FE-FPU-/110598558647?pt=LH_DefaultDomain_0&hash=item19c03013b7

And here is lots of 24 pieces for $600, or $25 each:

http://cgi.ebay.com/Motorola-XC68040FE33M-CPU-qty-24-NEW-/110598556270?pt=LH_DefaultDomain_0&hash=item19c0300a6e

Anybody interested in a group buy of these? I don't have $600 handy to drop on a whole lot, but can put $100 in. Who else?