Awesome! Aluminum is hard to come by around me unless you’re buying a truck load.
Try onlinemetals.com or even Amazon. I use them for chunky stuff for my mill.
BTW, onlinemetals has monthly sales & discount codes you’ll get via email/text after your first purchase.
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I checked the price of this chunk of aluminum vs Online Metals, and it’s quite a bit cheaper. It’s a good deal if you can make your projects work out of a 2x4x6 chunk.
I used to buy a lot from OnlineMetals when they were based in Seattle (and am still using a lot of stock that I bought years ago when they had a surplus section), now I usually find cutoffs on eBay. A lot of my work as a hobbyist can take use of what is trash for everyone else.
I recent example (more lathe oriented) was dozens of 2-6 inch long chunks of 2" diameter Delrin for $20 shipped. I can make a lot of use of those short pieces and it was way cheaper (about 10x cheaper) than OnlineMetals.
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Exactly, online metals is twice the price before shipping.
I think it depends on how close you are to one of their main distro centers. I used to have one that was 20 miles away and they competed well with other local metal supply stores. I’m still doing okay in NC because they have a center over the border in GA but I do try to use closer more local ones. It’s nice to have delivered stock -;easier & cheaper for stuff to feed a mill vs the plasma tables 
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Thirding the 4th axis comments…
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Online metals wants $66 a foot for the same size. That’s $33 for the same thing Langmuir is selling for $25. I have to pay shipping either way so it’s quite a bit cheaper. If I was closer to Dallas it would save me money, but it’s 4 hours away.
I don’t suppose you have an updated cutting parameters chart made up for this extender?
I’ve been following the chart pretty closely without the extender and you guys did a really good job with it, but WITH the extender I can’t seem to get even close to what that reduction % is. I blew apart a few 1/2" 5 flutes cutting some “Low-Carbon Steel Sheet”, that were running flawlessly before. It was nice to not have to adjust the spindle height, but after a few cutters, I’m not sure it’s worth it.
Even with the extension bottomed out in the spindle, I had to drop to nearly 50% feed rate to eliminate vibration. I was facing aluminum with a 3/8" end mill, 0.020" depth, and 50% engagement. I won’t use it with anything larger than 1/4" and the lightest cuts now…it really slows things down. But works pretty well on 1/32" drills and mills, as long as I dial out the run-out.
What are the odds you can release an updated version of the software that eliminates the speed limit? I’ve upgraded my power supply and motors and have a lot more power but I still can only go 100 ipm. 
I would even be ok with a warning popup that says “warning, this program contains feed rates that exceed the factory motor capabilities. Press ok to continue”
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The most I’ve gotten has been 180ipm on Y and 125ipm X. Anything above that on X I hit a resonant frequency on the gantry. For Y I maxed it at 180 due to seeing the full machine move on acceleration and deceleration. Most wont use more for cutting but it does help on tool changes.
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I have a Centroid Acorn board I just don’t want to hook it up because it’s for a different project and I don’t have a back up cnc. If I ever buy a bt30 spindle for the MR-1 I’ll go ahead and use the Acorn board.
I don’t know how much cutting I would do over 100ipm but when I have a part on the right side of the machine the time it takes to go from home to G54 feels like an eternity.
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I have rapid travels at over twice that speed. I found 300ipm to work well with the stock steppers (but Leadshine drivers) and can run higher with my Clearpath servos.
100ipm is so slow for tool changes and machine setup.