The equilibration buffer is mostly supplied by the company along with the TUNEL kit and it is standardized with other buffers (Tris-HCl (pH 6.6) in order to obtain pH 6.6.The recipe is given below (as per Promega make)

Equilibration Buffer

200mM potassium cacodylate (pH 6.6 at 25°C) 25mM Tris-HCl (pH 6.6 at 25°C) 0.2mM DTT 0.25mg/ml BSA 2.5mM cobalt chloride

Indeed, potassium cacodylate is toxic chemical and you have to handle with care.

If you do not want to use potassium cacodylate or need an alternative, then I would suggest you to go for One-step TUNEL Assay Kits for the detection of apoptosis. The few detail is given below:

One-step TUNEL Assay Kits

Elabscience® improves the process of the entire line of products, and offers you a highly sensitive, fast and simple apoptosis detection kit (TUNEL) at a lower price and a better product experience, which can better assist in apoptosis research on diseases related to cell function and research and development of related drugs.

Elabscience® one-step TUNEL assay kit is suitable for in situ apoptosis detection of tissue samples (Paraffin embedding, frozen section) and cells (Cell smears, cell climbing smears) in situ apoptotic detection. The test results can be directly observed through a fluorescence microscope.

The kit uses a new buffer that does not contain cacodylate, which is safe and non-toxic.

Hope this may be useful

Best