I'd suggest using a travel agent. They might charge you a service fee, but can hold reservations at the quoted fare for at least 1 day for each traveler while other itinerary details are finalized and/or the final purchase decision is contemplated.
Last minute bookings for a group of varying size each week and originating in different cities is not the type of business airlines will provide discounts/special amenities to attract.
As far as specific agencies, I would suggest a local (to you) travel agency that is small enough for you to develop a relationship with the owner/manager, but large enough to have the economy of scale to enable the ($25-50) service fee charged to warrant the time it takes to handle your bookings (likely this is an agency with a strong corporate travel department). If the travel agency is handling hotel/tour operator bookings, you might be able to get the air service fees waived, but as there is generally little/no commission to be earned on the air routes you mentioned, you will probably have to pay for the agency's services.
Eventually, group bookings may make sense, but these necessitate a commitment of 10 travelers on the same itinerary.
Good luck with the business!