Many optical things loose their purge over time anyway. It's rarely a catastrophe. I've used any number of cameras, night vision systems, etc. that are not at all sealed, so full of air. Most never fog inside anyway.
Mostly you can do fine with dry air. Seal it up on a nice dry day (either dead of winter or when the a/c is working hard) and it's fine.
If you want to do your own inert purge, argon is the way. Optically transparent (some other thermal imagers cooled with argon, so the entire optical cavity was full of it), much more inert than N2, doesn't leak out as easily, heavy which is nice for filling. You open a fill port or the end lens or whatever, face that up, fill it with argon and at your leisure put the cap back on.
Where do you get argon? Wine preservers. Serious wine snobs who don't finish a bottle keep the O2 from messing with it by squirting in low pressure argon. Bottles as low as $20 for several hundred fills. If you know a wine snob: ask them for a squirt for free though, even better!
