Please check the following PDF attachment.

There several sources.

For example, you can use molecular spectroscopy data set HITRAN

https://hitran.org

https://link.springer.com/article/10.1007%2Fs12200-010-0115-8

Dear Dr.Ali

The optical constants of methane and many other materials can be found on the refractive index database below. You can use them to calculate the wavelength-dependent absorption coefficient from the relation alpha= 4*pi*k/lambda

When using the data you should cite their original reference which in each page

https://refractiveindex.info/?shelf=organic&book=methane&page=Rollefson

Kind regards