How do you convert gallons of 50 percent caustic soda into dry tons of caustic soda?

12.6 lbs per gallon

Sodium Hydroxide (NaOH) A powerful base often used in industries.

Caustic Soda (Sodium hydroxide) can be found in Pharmacies/Drug stores such as Boots.

we can get it in most chemical shops or hardware shops

No, it is mostly prepared from brine (The saturated solution of NaCl) by electrolytic method.