I really try to not offer assistance that promotes my business here in order to stay within the forum guidelines, but if you are unable to secure this locally, I would be happy to help you. We have 2 1/4"x14"x6" in both straight and 90 degree angle boot. You can also get 6"x4" reducers to accommodate your opening.
I would first contact a local HVAC wholesale distributor and ask. They claim to not sell to homeowners, but most will turn a blind eye to a cash sale for duct and fittings.
If you have a problem with that, contact us at info(at)homeairdirect(dot)com and I will see what we can do to help.
Again, I do not want to have a problem with the forum rules here, and I do think you should try local first, but if to no avail, I will be happy to help. I could have just slipped my sight in as a source, but I think honesty is the best policy.
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