I remember seeing some on eBay some time ago...

What diameter do you need? Perhaps extending an existing cutter is an option. When I needed a long 1/2" ballnose cutter for MDF I just used a cheap router cutter with a 1/4" shank and made an extension with it using 12mm steel bar. Then hardened it for good measure. I chose 12mm steel bar as being a slightly lower diameter than the cutter the extension cannot interfere with what you're cutting.