Atmospheric solar storage tanks made of welded stainless steel (either 304 or 316) are readily available. Stainless tanks are fabricated based on the particular space and volume requirements of the system. It is also possible to use multiple stainless tanks manifolded together. This approach can give you the ability to use the combined storage capacity of two tanks that may not be able to be installed in a single unit. The disadvantage of stainless tanks are:
1) price – these are generally the most expensive of the atmospheric tank options although many times they are a fraction of the cost of pressurized large volume tanks
2) insulation – these tanks generally require insulating on site.