You can find silicon grease used in food processing machinery & in brake systems. So if you know someone who repairs the equipment in an institutional cafeteria or a sausage factory you can hit them up.
I've been using "kleen-flo eze slide brake lubricant", came from NAPA auto parts. It soaked on a Systema hopup rubber for 6 months prior to going into a mechbox and there was no swelling.
Airsoft, where nothing is hurt but feelings.