I'm currently installing the ducting for my new DC system and am using mostly 6 inch sewer and drain pipe. I've discovered with some already pointed out in that the places where you can purchase the straight pipe from at a reasonable price (I paid around $12 for 10 foot sections) charge a lot for the fittings.
Luckily, I live in a state that has Menards stores. And their sewer and drain pipe fittings are very reasonable. If you live in one of the midwest states where they have stores I've found them to be the cheapest for these type fittings. As and example, the 6 x 6 x 6 Wye is $16.99 (https://www.menards.com/main/plumbin...009-c-9383.htm). And several times a year they have 11% rebate sales on everything in the store, so that's when I buy stuff like this.