Yeah, Gannon is right before the final battle. Brotherhood is the only way to get the training unmodded.
Places you can get power armor without mods:
1.
NCR salvaged. Doesn't require training, not as good.
2.
Remnants, from Arcade's endgame quest.
3.
Brotherhood.
4.
T-45d,
T-51b, Brotherhood
T-51b.
Really, I wouldn't bother. By the endgame light and medium armor is very comparable, and those don't slow you down. If I play through again I'm going all light + the faster run and critical perks for it.
If you must, get my mod set; it adds a few new variants and other ways to get training.