A lawyer won't expedite the process all they can do is make sure that all your paperwork is in order.
See the links in my signature, below, for information on bringing your fiance(e) to the US.
If you are still insistent on having an attorney then you will, preferably, want one that is AILA registered though that is still no guarantee of proficiency.
Other forums you may wish to check out that deal specifically in US marriage-based visas are visajourney.com, diveintoamerica.com and britishexpats.com.
Edited for clarity!