Here is how you get it at any level without doing Arcade's quest:
Alright, here goes. You're gonna wanna have a stealthboy. More than one would be good, but I managed it with just the one. You'll start off in the Legion camp "Cottonwood Cove". In case you don't know where that is, just follow the highway East from Nipton to Searchlight and from there all the way to the Colorado River. Once in Cottonwood Cove, go due East across the river to the opposite side of the bank. SAVE. From there, follow the easterly bank due north until you start seeing red tics on your radar, or you see a large, pronounced ramp headed to the top of the cliff-face. This is an unmarked location called "Deathclaw Promontory". Fun Place.
Chill out in sneak until you're "Hidden", then SAVE and activate your stealthboy and slowly work your way up the ramp. It helps to do this at night, but again isn't necessary. The deathclaws will see through your stealthboy, but it at least gives you a head start. At the top of the ramp, go immediately right and bunny hop your ass up off the "dirt path" and onto the jagged rocks as soon as you can. You'll likely be detected the moment you reach the top of the ramp, so this might be a mad dash type situation. Jump your ass off til you are as high on the jagged rocks as you can get. At this point, you should be safe, and won't need to sneak anymore. SAVE.
Keep the Colorado River on your right as you work your way along the rocks going South, paralleling the dirt path on your left. This route should dead end into a sheer rock face. Once you reach the rock face, snake your way left, still along the rocks, and try and perch yourself on the shelf of rocks on emanating from the rock face. This might take a try or two, and you don't want to fall below, so SAVE beforehand. (The trick to making this jump isn't to jump from the rocks your on onto the shelf, but rather to jump straight against the cliff face, then hold forward to "grip" the surface, and walk "against" the wall a couple of feet to the left and spam the jump button til you get on the rock shelf. That might be a shit explanation, if so I can try and clarify further).
Once on the rock shelf, you shouldn't be able to progress any further without dropping down to the dirt path below. You'll know you're in the right place if you go to the Northeastern most point of the tiny rock shelf, and look to your right. The dirt path below should snake off to the right and into a crevice in the canyon wall. SAVE YOUR GAME.
Alright, here is the ball-tickler, folks. While on the rock shelf, have your back facing the sheer canyon wall and look forward. There should be a triangular pile of rocks in front of you, just outside the mouth of the crevice. On the far side of this rubble pile are the bodies of two dead prospectors, one in T-51b and the other in Advanced Power Armor (suit only, the helmet can be found in Silver Peak Mine on the road to Jacobstown).
SAVE, as this will take several tries. Seriously. Hop down off the rock face, locate the bodies on the far side of the rock pile and boogey your ass back onto the nearest rocks you can find. Next, just get back to the rocks you came here on, and find a way to jump, or "climb" down the cliff and get into the Colorado as quickly as possible (Deathclaws don't swim, ya'lls).
This should help:
The first red X is where you should start sneaking, the second where you make your mad dash from the ramp onto the rocks and so on and so forth.
