There are plenty of PHP/MySQL specialists out there whith more than just a passing familiarity with Dreamweaver.
However, I think you'll find difficulty in rooting out someone with a worthwhile level of Mambo server admin experience.
Not too many people will have that combination of skills and once more I suggest you look into web designers local to you rather than use a help forum for recruitment purposes.
I've used Mambo, but not nearly as much as other Content Management Systems and what you must remember is that any web specialist worth the name will either have their own prefered CMS (they may have possibly written it themselves) or will be expecting to work with one of the mainstream products offered by most hosts.
You don't mention several vital factors:
1. Are you serving your own site from your own server or from a managed host based server or are you managing your own server with a dedicated host, or....
2. A rough guide to the current size of the site (no. of pages, no. of databases, traffic levels currently enjoyed...)
3. Are you using secure financial transactions and if so how are they being processed currently ?
There are loads more considerations to be made which I'm not going to go into here since I still don't think this is an appropriate thread for a helproom.
I'd get the Yellow Pages out and start looking at web design houses in your locality, or tell us which area you are in since a few of us who post here may be able to recommend someone in that region or do as suggested in another thread and check out your local college or university and see if you can recruit some help from a willing E-Commerce student group.
As I've said, finding someone who knows Dreamweaver combined with PHP and MySQL is the easy bit. Finding someone who can just walk straight into a Mambo CMS setup on your server may prove a little more difficult.
Sorry about being a wet blanket, but I still say that the function of this forum is to offer help.