its hard to find used PVC, at least from my experiences, probably because it breaks, has no recycle value, and is considered cheep by those who work with it, not worth storing for future potential use.
wood and metal however are often stored for repeated use. 1" thick boards of certain woods, that are not too old or brittle, can be bent to make the hoops of a hoop house, and doubled up to reinforce it for semi-permenent use, although 1 1" board is probably enough unless you can rediculous winds and/or snow drifts up the side of your hoops.
you can find 1" boards all over the place. they are used for decking(be careful though, sometimes pressure treated, dont want splinters or to breath in this dust, or get dust from it in your growing area) and siding. check your local scrap pile at the transfer station, or just call around siding and porch building companies to see if they know any scrap lying around. chances are they have a pile somewhere, or have a client or colleague who does.
metal poles are a bit harder to come by, especially to find a bunch of them all together of uniform sizes. the obvious places include plumbers, recycling yards, transfer stations, who may let you pick periodically, and its just a matter of time before you have enough piping of the sizes you need to make whatever size hoop house you are looking for. another option is broken pole construction being replaced, like car ports, pop up canopies rented by party suppliers, and other greenhouses damaged due to age or storms or whatever being scrapped, and replaced by new stuff. calling companies that deal in this stuff might open up some sources for free or cheep materials.
another is military surplus. you can sometimes find pallets of uniform steel or aluminum pipes, as well as wooden posts for military tents and plumbing, being liquidated. this is usually done via auction, and you need the means to pick up what is usually a massive amount of materials all at once. you need to prove to them you have the means to pickup the stuff as well. which can be a barrier for those of us who just want to bid on something, and then figure it out, conscripting friends, or renting a truck or whatever.
anyway, these are some options. im sure there are others. the plastic sheeting doesn't often last long enough, nevermind being taken down and put back up. folks dont generally even think of future use, and dispose of it when they are done with it. but if you can find used plastic, even if its beat up. its better then nothing. letting construction companies know youd like to repurpose their plastic sheeting if possible could raise some oppertunities for repurposing plastic that might otherwise go to waste. also staying in touch with local hoop house farmers, so when they plan to redue their plastic, you could ask to repurpose their old stuff.
No Excuses Suburban Garden
300 total sq. ft. intensively cultivated, hugulkultur beds, producing at over 500lbs or $1500-5k of food annually in Zone 5b.
6 chickens worth of droppings, bedding, and 3 peoples worth of food waste provide plenty of compost to keep the beds growing indefinitely.
$500 up front cost(minus reclaimed materials): includes loam and compost, lumber for building beds and chicken coop, tools, chickens, and fencing for garden and chicken run, rain barrels or buckets
$300 upkeep costs(minus reclaimed supplies): includes seeds, chicken feed, mulch and water.
I use 1 3/8" steel tubing, it comes in 24' lengths, commonly used as the top rail in a chain link fence.
When I roll it into a half circle it's about 8' tall and 16' wide, when you use legs ( 1 5/8" ) you can raise it up 4' or so for a 12' ceiling.
The rolling machine uses three pulleys, two are driven and one is used to put the roll bend in the tubing. Have built dozens of hoop houses with it.
The local scrap yard gets 30 cents a pound for the tubing, I have picked it up for free just for hauling it away. Good luck.