What you're looking for is a mechanical face seal, which are used to allow rotating shafts to penetrate into a sealed volume such as a pump, mixer, compressor etc.
One important thing to remember about these seals is that all such seals leak to some degree. The exact seal you need will be driven by your specific requirements such as:
- How much leakage of the process fluids can you tolerate?
- What is the process fluid?
- What are the pressures and temperatures?
- Operating speed
On the higher end are engineered and stock mixer seals and cartridge seals like these from Flowserve. Those can easily run into the tens of thousands of dollars depending on your specific requirements.
There are also less expensive OEM seals designed for things like swimming pool pumps. Such as these, also from Flowserve. Again, depending on the specifics of your application you might get away with a sub $100 solution.
At the low end of the scale is simply using one or more radial lip seals that ride on the shaft, usually on either side of the bearing for the shaft.
You can also eliminate the need for a seal completely by using a magnetic drive mixer like these from Magnasafe. I have never seen a hand operated model though.
I can give you further guidance, but not without more detail from your end.
If your project involves any significant gas pressure, I strongly suggest that you following the ASME code for pressure vessels or contract with someone who is qualified and incorporating safety features to limit the possibility that the vessel could become over pressured and turn into a bomb. People continually underestimate the energy contained in relatively small volumes, like 20l, of pressurized gas and the consequences can be lethal.
Disclosure: I used to work as an product development engineer in the Seals division of Flowserve. I cited their literature because I'm more familiar with their products.