I am concerned about solvent vapors and a normal painter's mask being inefficient at blocking them. I noticed that the vapors permeated the small space even with sufficient ventilation. I have a better plan for ventilation this time, but has anyone found it worthwhile investing in a better mask?

Sure if you feel like it, wont be bad to invest in a better mask

Or, instead, maybe it's worthwhile to invest in a less harsh solvent like limonene?

Both are very good answers thank you.

If you can't modify the environment by providing enough ventilation to lower your exposure then a mask is the right choice. Make sure to use one that is for organic vapors. Might be a good idea to use glasses as well as the vapors can also be absorbed through your eyes, clear shooting (marksman) glasses work perfect.

Is limonene sold in department stores in the US or does it have to be ordered online? And can it be used as a substitute for naphtha in nomans tek?

It can substitute but you cant freeze nor evap it, you have to salt out. Check BLAB tek for more instructions.

I dont think youll find it in department stores, it is usually ordered online but its not at all a watched product.