I build liquid rocket engines, and their fuel refinement systems; in a manner thats totally legal for normal (US) civilians.

for instance, did you know that your house's natural gas line is usually ~80% (varies by municipality, lowest I've seen is 74%) methane by volume, and is actually shockingly easy to purify and pressurize into usable methane for a methalox cycle engine?

its even easier to make LOX (liquid oxygen), all you need is: air (in vast supply here on lovely Terra) literally any cooler design that gets down to -183C....( the Glifford McMahon cycle works; and you can build that at home), a fractionating column, (you can buy them off the internet, you'll want metal, but glassware is still fine-ish as long as you account for thermal shrinkage....and never ever let it heat back up to room temp), a cryo-rated compressor (sold on ebay), and a dewar (also on eBay)?