I am in Sacramento area but I used to get them through AIRGAS, the welding supply. You have to lease the bottle but it was cheap. I think it was about $20-25/month. You need a regulator and adapter too. Run about 40psi.
Once I got a second machine and a service contract it was cheap enough to get the zero air rebuilt.
People are usually shocked when they find out I'm not a very good electrician.
If you go with the large "BAR 97" zero air cylinder, it could last one year or more
depending on how many test you do.
Air Gas has a monthly Haz Mat fee of about $7.50 plus a "daily rental fee".
It adds up to about $16 to $20 per month.
I get a whole year (about 2500 smogs and 100 calibrations) out of the large zero-air bottle on my ESP Gen 1. I close the valve every night, and open it every morning (except Sunday), rain or shine. The N2 bottle only gives me a few months ... I think there might be a tiny leak, because the pressure reads zero in the morning, while the zero air pressure is still holding at over half.