we make that stuff in-house:
(not trying to be an advertisement, but any sign shop that has the ability to custom manufacture aluminum signs could do this for you pretty quick)
*welded extruded aluminum tubing with 1/4" welded&drilled aluminum mounting plates.
*toggle/expansion bolted to ceiling and you're set!
however, if you're dying for a DIY fix then get some 1/4" extruded aluminum/steel angle, can either be the length of the entire sign OR simply use 4" sections placed across the sign:
also noted, that, if you're doing this for a parking ramp... better make sure there aren't steel cables run throughout the concrete ceiling (supports the structure) where you'll be drilling. You'll know if you hit them, because you'll hear a gun-shot SNAPping sound and it's HUGE $$$ to fix.