Pioneer Supplies' stock is based on RNG, so that means that you either have to be lucky to find stuff or use some tricks to have better odds. The following method is one of the best at the time, perfect if you're looking for high grade suits or weapons.
IMPORTANT NOTE: even though vendors have always something to sell at any given time, upgraded equipment is restocked only after every Power-Play tick (Thursday morning UTC), after servers go down for weekly maintenance. Don't bother hunting for gear on Tuesday or Wednesday; the best time to do so is right after this tick.
That said, here's the method:
Step 1: use any third party tools that have a price update sorting system; EDDB is perfect for this, so we'll use this as an example
Step 2: start sorting stations by oldest price update;
Explanation: since these updates work from player data gathering, that means the older the latest update, the less players have been to the station, the more the chance their Pioneer Supplies' upgraded gear hasn't been already sold.
Step 3: head over to the station and check their suits/weapons stock; if you've been unlucky, try again, maybe note the system you've visited
Step 4: PROFIT!
Pro tip #1: since other people might be trying this same method, don't use the first results you get with the third party tool; instead, move a couple of pages forward
Pro tip #2: if you come across an upgraded suit or weapon that you already have, contact other friends or members of your squadron to see if anyone else is interested; Pioneer Supplies stock should be universal for all players.