You may just be looking for a name but here I am with a mile high perspective on this. The resource for this is not a company with sales people sitting around waiting for the phone to ring but someone who is on a job right now doing an implementation.
You will have a better experience if you go with an individual who is dedicated to you for a period of time and who teaches you how to administer it. Companies are less likely to be very dedicated (typically highly skilled people will sell the job then lower skilled people will come in to try to do the work) and less likely to teach you how to take care of it (they count on ongoing support revenue, and will often undermine and/or discredit the IT people that are on staff.)
I'd post ads on DICE.COM and CRAIGSLIST for an independent consultant with 10+ years experience who has done a recent Exchange 2003 implementation and who has an EXCHANGE 2003 MCSE cert. Then make sure you talk to at least THREE of his recent references and preferably go and meet with them on site to check out his work. (Make sure he is referring you to high level people and not his buddy in the mail room.) You could expect to pay $50 to $100+/hour for this person and would want him on site full time for as long as it takes, probably six to eight weeks, to do the roll out.
The up front cost may be the same as a company but this will get you and your company much farther along and much more independent in the long run. You will get great training and make some great connections with other companies.