Off the top of my head I know I have 4 complete sets of X-01 hanging out in my castle fortress, but I have parts to a 5th and 6th, if we're simply assuming head and torso here.

I've found one in the big dig, the tunnel you go into with Bobby No Nose, the custom tower has a full set at lvl 30+, there is a partial set (well a pair) one under the underpass near quincy next to some military vehicles, and out in the swamp next to a down'd vertibird.

The Glowing sea as a partial set if you follow the left side of the map and hit up the abandoned shack, this is lvl dependent as well. I found another partial set along the road leading into the glowing sea, I'll update the name tomorrow.

South Boston Military Checkpoint has a full set lvl 30 plus.

So far no trader is selling the sets, tomorrow I intend to use the power armor guides and find all the frames, and hopefully since some frames that claimed to have x-01 didn't, we'll see if there are ones that do.

I sorta have an X-01 problem...

On top of the 35 Court building is a complete set guarded by a Sentry Bot and an Assaultron. I'm pretty sure it's X-01 regardless of your level but I haven't tried at lower levels.

every now and then rowdy at the atmo cats will have x-01 pieces. and as far as the sets spawning depending on level i call bs. i looked for power armor sets well after lvl 30 and only found 2 full sets and 2 near full ones. ive been to every location said to have x-01 armor.

After 3 play throughs, at levels 31, 33 and 34 I've always found a full suit of X-01 on 35 court.

Well I took a set of T45 on this playthrough which was X-01 on my last playthrough. My girl is about lvl 12 this time and the previous character was 34ish last time.