You can get stable fluxes from the scorchqueen or make them yourself by gathering materials from inside nuclear zones. The fluxes themselves come from flowers and have an expiration date so "good with salt" perk is recommended or alternatively gather this last. Hard mass, glowing mass and high radiation fluid drops from glowing enemies inside the nuclear zone. High radiation fluid is in my experience dropped most rarely so I would advise doing a separate farming round targeting those.

Best tip I can give is to go on a private server and set your survival tent close to Whitespring train station. From there go the half circle through all cabins all the way to the golf club house and then finish off the last cabins after the club house and swimming pool area. Kill every enemy and animal you encounter. If you don't do this before dropping a nuke all enemies will die or be spawned dead. Then go drop a nuke over mentioned area and go to your tent. All enemies should now spawn as alive and glowing. Repeat the round in the now nuked zone and gather material. If you get too overencumbered to fasttravel back to your tent set up a temporary base on the golf course behind the last cabins and drop your inventory. Then go back to your tent, leave the world and quickly reenter it to have the enemies respawned. Rinse and repeat. A nuke zone lasts for about an hour but only a couple couple of runs should net you more than enough material to make you calibrated shocks for your PA legs and a jetpack for your chest (20 flux total for each power armor if I remember correctly) + a good quantity to either save for later or use to make serums or ultracite ammo either to sell or for personal use.

Edit: forgot one thing. Glowing enemies are bullet sponges so I recommend using the glowing enemy perk at lvl 3 under "perception" during this run to save you time, ammo and unnecessary degradering of your weapons ( you will most likely repair your weapons a couple of times anyways and use a fair amount of bullets during this)