As far as I know Plesk is only available directly from SW Soft, it is cPanel which you have to buy from a reseller.
One way of getting it cheaper would be to buy a VPS license for Plesk. You could buy a very large VPS which would preform at almost the same level as a dedicated server - but this depends on your needs.
Also don't forget that with Plesk you can pay for the number of domains you want. If you only need it for small use you can buy a 1 or 10 domains license, medium use 30 or larger 100 or unlimited. If you need a larger license then you can probably afford the extra