What are alternate options that one can use as a sleeping pad if he/she doesn't have one along?
P.S.: Lets assume that he/she has a tent. And we are not talking about any sorts of extreme weather and altitude.
Depends on the situation and therefore the gear you have along and the environment you are in.
For example if you are on an alpine tour, you can do similar as we did this year in a bivouac:
Piles of clothes (though not that comfortable) they do help insulate you when it's cold (one of the main things a sleeping mat does is insulate you from the ground to keep you warm, not just make it more comfortable).