Terry Tietjen. ;p
He's a local trainer, but that's not much help, I'm sure.
But, seriously, with a little creativity, you can do just about anything without a round pen. Just pick a trainer you like, and look at what (s)he's doing with the round pen. Then, do some brainstorming. If the round pen is being used as a point of reference, lay out some long rope or hose and use it as your 'round pen'. If they're just using a portion of the pen as a barrier (for, say, sidepassing), use a portion of your existing fence, or the broad side of a barn, etc.
I have a round pen, but it's really small. One thing I ended up doing for Bandit and Dante was, I got some plastic barrels from where my dad works, drilled some holes through them, and ran electric wire (without electricity!) through the holes, making a temporary fence that (if I remove the wires) doubled as a barrel racing set up. It also teaches the horse to respect barriers better -- at first, Dante always tried to duck under the wire, but then I taught him that it was harder work to be outside that pen, and he never ducked it again (he usually doesn't duck the actual electric fence anymore, either!)