Online Travel Booking and Bidding Agencies - looking for a basic agency to do this:

In the old days, I'd pick out a flight, call my travel agent, and they'd reserve it.
Then I called my client, and they would call same agency with a credit card to pay for that flight.

I'd now like to reserve the exact flight I want on whatever airline, and then all the client does is call someone with a credit card to pay. This worked great until today by booking with airlines, but as of today United no longer holds flights - you must pay immediately, so this plan is off.

Any online agencies offer this?