You can expect a few weeks to get one made by Dexter and shipped out. Not too expensive for one of the most critical components of a trailer. About $375 for an
axle with
brakes. You may find something in stock someplace, but every Dexter dealer I've ever spoken to gave me the two week estimate.
And getting it aligned correctly is important, so if you put it on yourself, I'd consult a trailer running gear specialist unless you're familiar with such things.
And yes, trailing arms stick back from the
axle and leading arms stick forward. But the same
axle can put used both ways. My axle came delivered from Dexter set up with the
brakes oriented for a trailing set up. Since the 13' Scamps use the leading arm configuration, my installer just turned the
brakes around and installed it that way.