Can I get this silicone grease at any hardware store? And what about these o rings? do I need them to convert a Shaeffer or just the grease? Is there a particular size that one needs of these o rings?
O-rings aren't needed, just the grease. Use just enough to fill the threads on the section completely, fill the barrel with ink, and screw the section in place, being careful not to overtighten or you may crack the barrel! The excess grease that squeezes out will wipe off nicely with a paper towel or tissue.
Most hardware stores will have small containers of grease in the plumbing area, with the faucet parts. Make sure what you get is marked as 100% silicone grease, as there is usually at least one other type of grease in the same area. Silicone grease is considered harmless to the materials from which fountain pens are made, while the other (petroleum-based) stuff can cause irreparable damage to some materials.
I recently put an Omas Paragon nib into an old NNS (ask me why LOL) and converted it to an eye dropper filler with just a dab of silicon grease. My word do they hold a lot of ink.