There is really no such thing as 37% formalin. The active ingredient in formalin is formaldehyde. Formaldehyde is a gas and is very soluble in water. When the water is saturated with formaldehyde it is 37% formaldehyde by weight. This is a chemical constant at normal room temperature and pressure. Formaldehyde in water is not stable and will degrade to into inactive compounds. Therefore it other compounds such as methanol are added to stabilize it. Formalin is often used as a “10% solution”. This means that the Formalin at “100%” is diluted 1+9 with water or more usually a buffer* to a “10%” solution. This is in fact 3.7% formaldehyde solution. A "37%" solution of formalin is possible but nobody sells it at that concentration.

* One of the breakdown products of formaldehyde is formic acid. The buffer keeps the pH in check.

As far was where to buy it; try a scientific or biologic supply company. I work in a research lab and we buy it all the time in gallon jugs. I am sure fisher scientific will supply in smaller amounts.

EDIT here it is

There is also an MSDS sheet there that shows you how it will kill you. But get real. Look up the MSDS sheet for sodium chloride. It is just as scary when spelled out in an MSDS sheet.