sheet metal shop, you may need custom work
make sure you have drawing & demensions of needs.
good luck.

Duct and clips are always custom made to order from a sheet metal shop. This is why square duct is so expensive. The only square duct I have seen in the stores ready made is vent duct for downdraft kitchen cooktops.

Look in the yellow pages under heating and ac wholesale. Ask about 3-1/4 x 14 wall stack. The wholesaler may not want to sell to you. Best to get a little knowledge of terminology and applications beforehand.

Sounds like you got wallstack, as Airsome said. That type of duct is sized to fit within the cavity between 3.5 inch wall studs.

Make some careful measurements, then go to this website to see what's available...

http://www.graymetal.com/

Page 16 of their online catalogue is a good place to start. Get familiar with the terminology and the names of the parts you want. Then do as airsome said and look under "AC Wholsale" or "HVAC Supplies".

Most of these guys will sell only to contractors. However, if you call first and ask for a quote on something specific, they probably won't ask for any proof when you show up, money-in-hand. If they ask if you have an account, say "just ring it up as cash". You can still use a credit card for a "cash sale".

Some advice when dealing with HVAC wholsalers...

- Make sure the parts you buy are what you need. The wholesaler will get miffed if you try to return any. A pro would just use any un-needed parts on another job.

- Remember that wholesales don't have to sell to you. If it comes to that, just walk away.

- learn the part names, terminology, and sizes available. These guys haven't the time to deal with someone who doesn't know what they want.

- Don't pretend to be a pro. They'll see right through it.

Hope this helps.

Marco

Super advice by daft punk!
Also, look for hvac dealers in your area. Some have no problem selling sheet metal items to the public.