It’s looking like Uline or Blicks will be my best bet, but both would require shipping and I’m wondering if anyone has found a local source for corrugated cardboard sheets in quantity. I’ll need around 200sq’.
Blicks seems to be the best option and is substantially cheaper than Uline, but they don’t carry it in quantity in-store, and you can’t order online for store pick-up.
I’ve ordered from Uline, but as you mentioned…the shipping. I was able to wait until I needed other stuff too and placed one larger order at once. FWIW, they have really good pricing on chipboard sheets (11"x17") so if you do use Uline and also use chipboard, that’s one way to spread the shipping cost around some.
How ‘clean’ do you need the cardboard to be? Like, can it have markings on it? If they can, then going to local grocery stores and asking for the egg boxes can be pretty good/consistent cardboard (the boxes the crates of eggs go in, not the crates themselves).
Got a link for the uline chipboard?
Edit: never mind I un-lazied and googled it.
https://www.uline.com/Product/Detail/S-6418/Corrugated-Pads/11-x-17-Chipboard-Pads-022-thick
Uline has physical locations, would sidestep the shipping.
I get mine from Amazon; haven’t compared prices though.
haha, glad you found it! Yeah, I don’t have any physical locations near me. Chipboard is great for making topo maps, and they have multiple thicknesses in 11x17" sheets which is great. Far cheaper per sheet than anywhere else I’ve found it.
I really like the corrugated cardboard from them too-at 11x17 it’s a great size for jigs. I ordered I think 50 or 75 sheets and I use it for all of my jigs now (used to have a hoard of amazon boxes that I cut out from and used before).
Can’t order for pick-up.
I had exhausted Amazon search first, but did not see these. How thick are they?
Same as Amazon boxes (not home to measure them right now, sorry). Yeah, they don’t come up if you search “cardboard,” you have to search for “corrugated.” Took me a while to find them, as a result!
In Cincinnati, I purchased online and picked up at BGR Packaging. 40”x40” and 20”x30”.
I was gonna say, “You buy cardboard?” Amazon deliveries keep me supplied. 
Well, it was super cheap and I was ordering other stuff from Uline…otherwise I wouldn’t have:)
you might try looking in your area for a fulfillment company. the people that create the packaging etc. for a specific product. my wife used one for a kit she produced for work. they might be a source. heck they might have stuff that wasn’t printed right and you can get cheap. they recover some wasted expense and you get cheap cardboard with no shipping.
I’d stop buying it from Amazon except they keep sending such cool stuff along with my cardboard 
Exactly.
If you don’t need it to be completely blank and you don’t mind doing some hand cutting, I’d recommend either Lowes or Walmart. Both sell corrugated cardboard boxes in my area at reasonable prices. I regularly by the Lowes medium size box, 18x18x18 (I think). I sell wood signs and I can use one of these boxes to ship four signs using each side. I think I pay around $1.50 each.
This may not work for what you need, but its super convenient and economical for me.
Thanks. I need them to be clear of print and flat.
FYI, Amazon boxes are .15 
They’re my go-to
Hi, I’m super late to this thread… but maybe useful to someone in the future:
I use the shipping flats from Amazon
For my first year i was also deconstructing and hoarding amazon boxes…it’s good cardboard
But they take up space, and take time both to break down and to cut to size.
The 12x20 Flats are perfect close in thickness to the boards i use, and 11g steel (When i’m sending the finals off to Plasma.)
They come in a neat stack. No dunnage (they are dunnage) just shrink-wrap and a strap. They line up perfect on the bed, and are consistent thickness
One downside in shipping the corners of the top & bottom 3 can be pretty smushed. There are two sellers i’ve tried from the first had crisper edges… i’ll dig up both links and share shortly