I've been searching all over the net for 12 by 12 inch rectangular ducting with at least 2-3 feet in length and as close as I can get was at mcmasters.com, which had 12 by 8. Every other site sells circular ducting, which I do not want. I was just wondering if people actually fabricate their own rectangular ventilation ducting or does anyone know a good place/site (besides mcmasters and grainger) I can look up rectangular ducting products that's 12 by 12 inch.

I've been searching all over the net for 12 by 12 inch rectangular ducting with at least 2-3 feet in length and as close as I can get was at mcmasters.com, which had 12 by 8. Every other site sells circular ducting, which I do not want. I was just wondering if people actually fabricate their own rectangular ventilation ducting or does anyone know a good place/site (besides mcmasters and grainger) I can look up rectangular ducting products that's 12 by 12 inch.

12X12 is not a real standard size for duct. Go to a local sheet metal shop and have them make exactly what you want.

Why not get duct board and make your own? 1 sheet @ 4X8 will give you a 12X12 8 ft. long.