Where can I find NMOS and PMOS which have finger structure to have higher width for MOSFET?

You just have to assign nf as a value larger than one. Just click on a MOSFET and press Q. In the dialog box there will be a text line containing the W, L and nf.

Use Maguc VLSI to generate multifinger layout

I wanted to have aspect ratios above 1000 for the design. But, Xschem is aborting the simulation for high aspect ratios. So, to have finger like structure which MOSFET should i prefer in skywater library. Also, in MOS properties, how to use 'nf' and 'mult'?

You need a combination of multiple FET (M=…) and multiple finger (NF=..) to get a large total W.