If you don't want to source it to someone with a wide enough printer I'd suggest ordering a roll of the WrapCut (there's a banner in the upper right for me right now but they're a MM). Print your pieces with 1" - 2" overlap whatever you're comfortable with having extra on the first try doesn't hurt.
Now the tricky part, lets say you're doing a sign 4' x 8' for the purpose of my explanation and you wish to have a horizontal seam and you have a 30" printer.
You're first panel is 28" with the gripper of your printer by 8' long, second panel would be same size. Now you have 2" of overlap @ the bottom so measure on you substrate somewhere between 28" & 26" and make a few reference marks. Now run a line of the filament tape along this line. Apply your top panel, overlap your bottom panel and then pull the filament tape.
You should have a perfect butted seam with perfect alignment if all went well. Hopefully that makes sense, if not I know I've seen an illustration of this technique and I'll try to scrounge it up for you!
-Nathan