Premium Domains

I work with a small firm, 20 computers, we have a domain where about 15 of them using XP professional are connected, the rest are Windows7 home Premium (which came with the new computers OEM). This has turned out a bit of a pain as these cannot be connected to the Domain, nor can they be accessed by remote desktop. I would like to upgrade these to Windows 7 professional including licences etc, but I don’t have a clue about the licence thing etc.
2 questions.
1) Can anyone in UK give me an address(url) of where I can purchase these fairly cheaply quantity= 5.

We will be buying 2 more computers
2) Where can I buy these computers with ‘Windows7 Professional’ already installed, licences included.

Any advice on this would be welcomed.

Thanks in advance

The best thing to do is to contact Microsoft and get a Multiple Licence
so you only need one copy and then can install it on those 5 machines.