do you know some webhosters that provide plone pre-installed?

I'm sure there are providers that would do it for you if you ask around. I don't specifically know of any offhand that install Plone, but again I'm positive you'll find a provider that will handle it.

Just ask one. I am sure someone would be more than happy to install it for you.

Looking on wikipedia (http://en.wikipedia.org/wiki/Plone_%28software%29) Plone seems quite complicated to get running, you wouldn't get it on shared or reseller providing this. I would suggest a VPS of some sort however you will most likely have to manage it yourself or pay a third party to manage it.

Check out plone.net (http://plone.net/hosting-providers). Plenty of hosts are listed that specialize in zope/plone hosting.