Yes, spare parts are plentiful. The Align 450 SA, SE parts fit the EXI with no problems, so you can use either Align Trex parts or Exi parts.
I have not had any fitment issues.
One of my helis has a EXI frame, with a Align swashplate, EXI metal head parts, Align Blade grips, Align Blue Gears, Micropro tail unit....
I even had one of the EXI 450's with a old MX400 head on it for a while.

That's one of the reasons I prefer the Trex/Exi 450 size heli. You can get parts just about anywhere.

They even sell a metal head/metal tail unit combo for $36 bucks.
http://www.hobbypartz.com/exicncmerohe.html
They are so cheap, I keep an extra set in my heli case. That way if I do have a crash...I just unbolt the head, swap it out, and keep on flying. I repair the other one later.

Now regarding compatiblility....
The Exi 450 is identical to the Align Trex SA version. The Trex SA, and SE stuff will fit the Exi.
The Align Trex Sport Edition has many simularities, and some parts will fit between the two. The biggest difference is the head block is wider on the Sport Edition, so you might run into some tweaking there.
As you mentioned the XL and X versions of the Trex are the older versions that are getting hard to find and are well on their way out.
The Pro versions of both are a bit different. Some of the head parts are compatible, the tail is not. The Pro versions have a torque tube (fancy name for shaft/gear driven) unlike the non-Pro versions which use a belt drive setup.

Personally, I hate the Pro's torque tube setup. On the belt driven version, if you get a boom strike...you just swap the tube. On the Pro version, a tail strike usually takes out the entire torque tube, and sometimes destroys the tail gears. Those torque tubes are WAY more expensive than a old 2 dollar tail boom tube. The other issue I have with the torque tube is if you clip the tail blades on the ground, or snag them in grass it strips out the gears. To replace it you have to tear down the entire tail gear box....it sucks. But some people really like it. Personal preference really.