In order to bend stainless smoothly you need to buy it before it is annealed.
Un-annealed stainless will often flake, so mills don't want to loose stock by it ageing in the unsold racks, and almost immediately anneal it.
Packing the pipes with sand is an old process and still used by some but the problem with it is that you just can't tape the end of the tubes closed. As you bend the tube, you displace the sand, so if you can completely contain it within the tube it has a high percentage of effect from keeping the ss from crimping on the inside of the radius BUT the radius still tends to flaten.
There are a bunch of stainless alloys available but generally speaking it does not like to stretch or compress easily. I'd guess because of the hardness that the chromeum adds to it.
The easiest alloy to work with is 409. That is what is currently used in production automobile exhaust systems and what will turn a redish brown as it oxidizes and it will eventually rust through.
It is not what people commonly think stainless is, i.e., a shinny chrome like looking metal that is relatively impervious to oxidation but will blue like chrome does when exposed to high heat like on stainless headers immediately after the exhaust port of the cylinder head.
You can buy close radius mandrel bent tubes that you need to weld on to the straight tube but then you are getting into the issues of welding stainless and not having the welds crack down the road somewhere.
It might sound like it is easy to work with on the surface, "but it ain't".
THEN, even if you are working with straight TUBING (not pipe) you are going to want to bead it so the hose you clamp on to it won't pop off. Don't even go there.