Victor,
We do not have a Pspice model for the LM136-2.5.
Would it be useful for your application to use a model for a related device?
If so, please consider the LM4030 or LM4041-N. These are precision voltage references which have Pspice models available at ti.com in their product folders:
http://www.ti.com/product/lm4030
http://www.ti.com/product/lm4041-n
Regards,
John