I guess it depends on your view of "reasonably priced", but NPD or Windsor-Fox are good sources (as well as most Mustang parts suppliers). I think you can generally expect it to cost around $130.
As alternative, you can make your own crossmember as we did. It just took some measuring, some drawing, some plumber's strap, and an hour to make a mockup model. Then we took it to a welder who welded one up for us for $35. The right price for sure.
With regard to:
65fastback2+2 said:
since the AOD is the same size as a C6. I suggest getting a junker C6 crossmember and using it, might take a little modification, but much cheaper
I never tried, but I'm pretty sure that a C6 crossmember will take quite a bit of modification. The AOD was only 3/4" longer than my original FMX (which the AOD is based on), but the location for the mount was 3"-4" different.
I just finished a 69 aod conversion with a 302. I did not want to make major exhaust changes, so I too made my crossmember. The Windsor unit would have required different exhaust pipes, so I made one. I wanted to keep my exhaust standard for now. All it took to make the cossmember was several pieces of angle iron all bolted together.