26 guage or heavier. (My local HD does stock some 4" and 6" in 26 gage, but as someone noted, only in 24" sections) The one thing to be concerned with around using HVAC wyes is that they are "backwards" for dust collection purposes, with the crimps on the wrong ends for the direction that air flows. You may still wish to source these from Oneida, Kenwood, Air-Handling Systems, etc.. for that reason. Large radius elbows may also be difficult to get without going to a DC supplier.
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The most expensive tool is the one you buy "cheaply" and often...