Any one know where I can find good FLAT ALUMINUM IN 304MMX304MM OR 12’X12"
How thick? Mic 6 tooling plate will be precision flat… But $. http://www.discountsteel.com/items/Aluminum_Sheet_Plate.cfm
1/8" or 3mm
I don’t know flat any material you’re going to find will be. But you can straighten it easily
not as easy as you’d think. . I have limited equipment i.e. no mill. and this will be my aluminum heated bed for my newest printer
I’ve had no luck with finding flat aluminum either, so i’ve given up and am now using 3mm glass. Heating isn’t as even as with aluminum, but it works well and it’s really flat
If you don’t mind the extra thickness and heat up time. http://m.ebay.com/itm/310575618025?nav=SEARCH
I use dibond for my parts @D_Rob , I have a 4x8 sheet and access to a cnc machine. I can cut you something for a small fee. If you are interested?
I have a slightly off milled finish plate now but when glass heats to 120c or so the glass will conform to the plate. introducing all the imperfections. just like it warps if you remove it hot and you have to heat it on the alu to get it flat enough to clip or it’ll break 
its how Ive broken the two 12 x 12 window glasses I have had break
@D_Rob stop using window glass its not designed to be heated.
@D_Rob Where are you located?
Might be able to help you out - we often have to press aluminium plate to get it “flat” for customers jobs at work, I will no doubt need one soon, so I can probably knock something up for you?
Or you could potentially shim it with aluminium foil? I do that sometimes…
@D_Rob I recently got 3/16 neoceram glass as a drop cut from a local glass house. PERFECTLY flat. Unfortunately not the full size of my usable build plate. But it works for now. No warping like 3mm boroglass with the larger build platforms. No breaking like window glass. I have shattered 4 window sheets from crowning of the aluminum plate due to my large build platform).
Neoceram has a working temp of 800f. Check around town for some. Its super pricey cut to order (like $200 pricey). But I got mine for $25 as a drop cut with 16" x 13" dims.
With the right press and a good straightedge, you can straighten a plate like this within .001. I do it all the time in my shop.
You might see if there’s a surplus aluminum scrapyard nearby. Ours handles both scrap and virgin, the scrap goes for about $2.00 a lb.
@Jarred_Baines mississippi, you?
Yeah, Australia (mate) 
So I’m sure you’ll have more luck on price sourcing it locally…
Like @Bob_Roth says, someone with a decent press should be able to flatten the plate very well, but that’s only if it’s bent, if it has surface deviations (scratches / gouges etc) it won’t do anything about them.
McMasterCarr.com has diamondplate for reasonable pricing (because it’s very popular), but I suspect flipping it over and using the underside wouldn’t work as the diamond plate pattern would cause uneven heating (or worse, act like a heatsink and radiate the heat away)
Plus its probably not very flat!
I guess this is why most of us are using glass.