If you have neutral buffered formalin, that can serve as an even better fixative than either formaldehyde or glutaraldehyde. Formaldehyde or paraformaldehyde also works, but please check the concentration needed. For SEM, osmium tetroxide is not needed but you would need to gold coat your samples for contrast enhancement.

Dear Najmin Ahmed thank you for posting this interesting technical question on RG. As indicated by Nabodita Sinha formaldehyde can even be a better fixative than glutaraldehyde. For more information about this please have a look at the following useful study in which the two fixatives have been compared:

Optimization of scanning electron microscope technique for amniotic membrane investigation: A preliminary study

Article Optimization of scanning electron microscope technique for a...

You can freely download this interesting article as pdf file because it has been posted by the authors as public full text.

I hope this helps. Good luck with you research and best wishes!