The recommended path for you would be to install a seperate box with windows 2003 and exchange 2003.
Migrate everything, and kil the old 2000 Exchange and DC.
Change operational leven of the domain AND the forest to win2003.
Install a third box with exchange 2010 on a Win2008R2 64bit server.
Again migrate everything and kill the old server.
Agan change the operational level of the domain and forest to 2008 r2 level.
As always, remember to move the FSMO roles, and allow a nice bit of time for proper replication, one hour should be fine, you can move some mailboxes while the AD replicates.
As far as the media goes.
You have some options...
If you have an MSDN / TechNet / SPLA agreement with Microsoft, you could just grab the 2003 medias from there, seeing as its a temporary installment, there would be no harm in using these.
Or just borrow the media and license from a friend temporarily, no harm in that...
Lastly, you could just download a torrent version, and get it over with!
Again, it's temporary, like a day or two max, so no biggie, you wont go to jail.
-Michael
