The other option is to use cords.
Sony's own TTL cords are roughly $40 per meter. You'll need at least one FA-CC1AM for the first 1.5 meters, and one or more FA-EC1AM cords to reach beyond that in 1.5 meter increments.Or
, you could use the PC sync on the a900's front and run a PC cord over to your flash (there are excellent DIY options
, too), then adapt it to your flash gun again with a Seagull SC-6 hotshoe adapter. The down side is that you're limited to manual control this way. The up side is you learn how to control lighting more thoroughly and get the most out of your fabulously powerful flash. I explored this option more thoroughly in this thread: Subject: Flash Cables