Trying to get Sentinel pieces as early as possible, as I prefer the look of standard and sturdy armor pieces btter than heavy ones. I've tried the national guard training yard, but the effect hasn't shown up once. Is there some place it shows up more often?

Its random generated though may be level specific.

I dont use the save method, i just reduced the cell respawn times via the console and have farmed over 500 legendaries so far. I find sentinel quite a bit.

In my limited experience using the save method, i was unable to find specific variants i wanted so limiting yourself with this method might be the issue.

Well, I'm on PS4, so console commands aren't an option for me.

Just to help you out in showing which items will likely drop with the sentinel prefix, here is every sentinel item i've found after killing 915 legendary enemies. For some reason i have found them much more often at higher level, though ive also been going after gunners more often too. They may infact be totally random or have some weighted factors. I can't be sure because the possibilities for drops are large due to high number of variations.

Nevertheless some prefixes only fit on certain armour parts, eg 'sprinter' on legs, 'fortifying' on chest and arms.

http://i.imgur.com/ZxsyPpa.jpg