Lets eventualy start this CNC - MKI

[routercnc]

I can't help with profile suppliers but you can cut those shapes with basic tools. Cut the aluminium on a bandsaw close to size and then finish the edge using a hand held router and carbide cutters. You need a template or straight edge to run the router edge along. Your shapes are main rectangles or triangles so should be OK.

As for 10mm vs 20mm I know aluminium is expensive so you are trying be economical so here are my suggestions on the minimum areas to use 20mm on, and why.

Z plate
as Dean says. Don't use the rails to add the stiffness. No worries about weight either - the steppers will lift more than you think once they are driving a ballscrew.

Y axis upper and lower bearing plates
Several reasons:
1. If you use 10mm plate you will have to use very small bolts into the side to mount them to the Y plate. So could probably only fit say 6mm bolts to connect them and this is just not good enough. They will bend and you can't get much torque into them to get a good clamping load

2. The 10mm plate has a small footprint onto the Y plate so is much more likely to bend up and down at the joint.

X axis bearing plates
1. They hold the gantry and stop it rotating. The triangular plate on the sides transfers this load down the outside to the bearing plate, but the X bearings are in the middle of the plate. This means that the unsupported corners on the inside will bend. Therefore they should be 20mm.
You could add another triangular plate on the insides as well but it is a bit awkward to attach back to the gantry front face.

2. You will need to counterbore the bolts which hold the X bearings onto the X bearing plate so that the gantry can sit flush. I'm guessing these are around M6 so need ~6mm of counterbore depth. In 10mm plate this leaves just 4mm of metal holding the bearing to the plate. This is a bit thin in my view, whereas 20mm still leaves 14mm of metal.

3. The triangular plates on the side will bolt onto the edges of the 10mm X-bearing plate and you will have to use fairly small bolts (~6mm) which is a bit small.

Finally . . .
X ballscrew drive plates
The strips which drop down to mount the X ballnut to are quite thin. They will twist when the load is applied as it is offset. A thicker plate with a pocket for the ballnut would be better, or at least some support, but it sounds like you have limited tools so you could build it like this and use the machine to make some beefier parts.

Another finally . . !
I don't think you can assemble the Y axis and Z axis can you? I think you've done the classic 'lock-out'. Walk through the steps of bolting it together in your mind and make sure it is possible and there is a build sequence.
As you are using packing pieces to mount the Z bearings to I think you will have to extend them (and the Y axis plate) say 15mm below the level of the lower Y axis bearing plates to be able to bolt from the back. The top is OK as it already extends above the level of the upper Y axis bearing plate so you can bolt through from the back.