there is a difference between a heating contractor and a heating supplier.
A heating supplier is the cheapest place to get most generic ductwork, however most will only sell to contractors.
a properly equipped sheet metal/HVAC contractor's shop can produce anything you could possibly need. Cost will depend on the contractor and the product but will run from a bit more to 50% more than the wholesale supplier.
The big home improvement box stores can actually be the cheapest place for the DIY guy but you are extremely limited on what's available.
If I were you, I'd either buy what I can at Home Depot (or equivalent) and pick up any custom stuff from one of the local sheet metal fab shops or get a price from the contractor to supply it all. Some contractors will blow you off, some will price you really high because they don't like people doing the work themselves but some will be willing to price you at a fair and reasonable level. I even offer free advice regarding sizing and layout but will admit that I won't spend too much time babysitting walk-ins as I have far more important things to be doing.
....and avoid flex duct if you can; the stuff is very restrictive.
Dave