Source for ‘mostly precision’ aluminum plate? I need 13"x13", approx 4 mm thick…while my Home Depot Diamondplate was cheap, it’s not exactly LEVEL…and I cannot tell if the diamond plate helps the rubber heater (by spreading the heat) or hurts (by reducing contact adhesion)
I got mine from discount steel, but any good local metal supply house should be more than capable.
McMaster-Carr? They have pretty much everything.
McMaster is always a possibility, but they stop at 12", and they’re far from the most price conscious. Discount steel looks interesting…I kinda want to take a gamble on a brake-cut piece of non-precision billet at $25…anything precise is closer to $65.
@Mike_Miller I recently went to my local steel supply and picked up a similar bed. I ask for 6063, and to be cut by waterjet. Not cheap, but intention was to end up with a perfect heat spreader. The next day went to pick up, much to my dismay they used a press break on one side, messed the entire sheet up. Didn’t purchase
I’m playing with something that I cannot attest to just yet, will after this weekend. I ordered a 12"x12" garolite LE plate, thin as I want to test it’s properties. I must say I am very impressed with how straight and level the material is, not to mention it weights nothing. Hope to know more after this weekend, how it handles heat and adhesion with different extruded plastics.
@Mike_Miller best is mic6 but its pricey and not normally available in thicknesses we need. I had mine CNC cut by a friend from a drop cut from discount for Eustathios. But on #Herculien I just had it pressed, and hand flattened it with a straight edge, a rubber mallet and a few beers 
I don’t know where you are but the cheapest place to get this would be a metal sourcer. Often they will have aluminum scraps which aren’t useful for machine shops but you could build a build plate out of them. For example a 12" x 12" piece is usually considered ‘worthless scrap’ and they get what ever the aluminum recycling price is for it.
Yes you can ask metal shops to review their scrap and just pull what you need. Many times, they’ll just give it to you.
@Mike_Miller up near boulder there’s a place called Western Recycle that sells scrap aluminum for $1/lb. Might be worth trying http://www.westernaluminumrecycling.com/
Here’s an actual shop: https://plus.google.com/109643418635122288896/about?gl=us&hl=en
Alas, I was even in Boulder today, but had to leave in a hurry to make an appointment. And I’m not normally that far north. 
+1 for Discount Steel. I’ve been totally happy with the 1/4" plate I bought from them. http://www.discountsteel.com/items/C250_Aluminum_Cast_Tooling_Plate.cfm
@Mark_Rehorst I want ‘flat enough’…mil spec is overkill.
Too small, I need 13"x13" or some serious mods to the frame of the printer…price is right, though. This http://www.discountsteel.com/items/5052_Aluminum_Plate.cfm?item_id=125&size_no=1 shear-cut, is $23…
To be honest unless the aluminum is the build surface, “pretty flat” for the heat spreader is good enough. It wasn’t hard to flatten my 17" x 17" heat spreader by hand. And the glass surface IS flat. Just devise a method to retain the glass to the bed that does not bow the glass to match the imperfect aluminum.
The spring binders are nice but tend to bow the glass since they only hold the glass at certain points. Also allow the bed to reach thermal equilibrium. When heating it will naturally cause a crown because the bottom is hotter than the top, and center is hotter than the sides.
Mic6 is awesome. It’s all we use at work for pattern tooling. But may be overkill since the glass is what you actually print on.
Just my 2cents.
Mill the aluminum plate to be level and flat, it will remove the raised portion making it a flat aluminum plate.
I’d LOVE to have a 13" square envelope on my mill. Unfortunately, it’s closer to 4"x6"
I just checked Home Depot and it seems that the cheapest Diamondplate they have is $19.15 - http://www.homedepot.com/p/Fasade-Diamond-Plate-2-ft-x-2-ft-Revealed-Edge-Lay-in-Ceiling-Tile-in-Brushed-Aluminum-L66-08/202654025 for a 2’ sq sheet.
They have all these other options which are actually cheaper and don’t have the diamond plate pattern.
http://www.homedepot.com/p/Ironman-8-in-x-16-in-Satin-Aluminum-Push-Plate-65592-0/203509446
http://www.homedepot.com/p/Schlage-8-in-x-34-in-Satin-Aluminum-Kickplate-678611/202504406
http://www.homedepot.com/p/MD-Building-Products-12-in-x-24-in-16-Gauge-Steel-Sheet-56070/205058577
I don’t know why you didn’t just get something without that as it seems that price is your reason for buying that.
None of the listed items are 13" wide. 
the whole point of this hobby, to me, is optimizing the price/utility part. Otherwise is just drop $5k on a makerbot+extended warrantee+build stand+…
And the first item listed is actually PVC.
I get that. 13"x13" was picked to permit the use of 12x12 glass build plates (giving the flat surface), not realizing that a 1/4"x12"x12" piece of glass is pretty much the lifting capacity of the belt system. (I discover this after buying two glass plates)
The diamond plate isn’t bad, it could just be a helluva lot better.
The point would be moot if I went with two wormscrews, letting me use the glass.