i've found 5" to be a little harder to find. mandrels are out there but fewer places carry 5". for most of the bends and the strait tube sections, i order from
Columbia River Mandrel Bends
they have reasonable prices and a good selection. i havent used any of the stainless stuff so i can't comment on that. they have many flanges and clamps as well. i used Amazon to search for the 5" machined v-bands since prices are all over the place and amazon seems to be the easiest way to find the cheaper offering. Burns stainless has a good reputation for quality, with an associated price tag. Reid Washbon racing exhaust has all the goodies in stainless too. i have talked with them. you pay for their stuff as well, but nice people and the product looks great. i will probably use them the next time i build a performance exhaust. they have 621 Inconel mandrels! super bragging rights if you build a TT manifold for a cummins with Inconel.
4" should pose no issues for locating parts. if you are looking for super tight CL bends Reid Washbon carries them in square radius and chassis shop sells the torus donuts in a square CL in 304l, MS and AL.
there is an outfit in denver that i can get the name and number for you if you are interested. they sell the v- band clamps and turbo outlet flanges(Mild Steel) for about half of any of the "performance" outfits online. i would bet they have 4" as well. quick shipping and pretty easy going staff.