Happyutopia,
I am sure people outside of the US book with a US travel agent specially if it is a "group". But also you can book everything with a Mexican travel agent. There are plenty of businesses in Mexico that do the "wedding abroad" arrangements, from tickets, reservations, ceremony, party, honeymoon.. everything! You do not mention where in Mexico you are planing to wed, but I am sure it is going to be in one of those turisitic sites with plenty of "wedding abroad" happenings. You shouldn't have a trouble finding some agent in Mexcio to make all the arrangements.