Actually, while hardware stores might sell pipe, usually it's in 10' lengths. Plumbing suppliers and steel suppliers would sell the 20' Sch. 40 black water pipe as it's called and you would need to know this because there are many types of Sch. 40 pipe. Most sales people at Home Depot for instance when you ask for Sch. 40 will point you at PVC pipe. 1.1/2" STD. CBW BLK A120/A53 PIPE PE otherwise.
I would stay away from Sch. 40 pipe for a fly system if I could help it however. Much heavier and less strong than stagebatten pipe designed for this purpose.
I don't work on fly systems anymore thus don't have pricing. Your local steel supplier can probably get it in or it would be stocked by your local rigging company. As for 1.1/2" (Inside Dia.) Sch. 40 black water pipe, it can be from $38.00 to $45.00 per 20' stick. Given most are actually 21'. Something you would also need to determine is if you want it threaded or not which will add to the cost. Batten pipe I don't think is available threaded because you also really are not supposted to be using plumbing couplers to join the pipe. Instead there are structural clamps on the market for rigging.
Places like Secoa, Mainstage Theatrical, Grand Stage, Sapsis Rigging, Feir Brothers, Peak, John Sakash and other rigging suppliers would be sources for info on this as well as JR. Clancy has a good website with a wealth of information.
Not sure about the completely straight part of the question as to what it means.
Chains to hang the pipe is also a rigging thing and something especially not sold in hardware stores. Call me a purist but hardware store chain is not rated for overhead hang, nor does it have the tolerances or safety factor to be using on stage. Yes it's used all the time but as per Stage Rigging Handbook II, p.127, "Regular steel chain is brittle and will have a tendency to shatter under the stress of a shock load." Lots more on the subject of trim chain. The hooks on safety chain are rated for 1,000# SWL. After that, you can use normal chain as long as it's grade 8 and you use appropriate hardware for it. 1/4" Proof Coil is the normal term for batten chains. The above companies as with a few others would be prime sources for batten chains and trim chains.