If I need to find an MOSFET spice model with a particular length and width, where do I start?
It is unusual to seek a MOSFET model for particular values of W and L, except perhaps at the minimum allowed values of those parameters. Specifying the W and L that you want and using the vendor-supplied model is the usual route.

You need to contact TSMC, or whoever supplied the model parameters, to find out the applicable range of W and L for those parameters. If the parameters were extracted with the intent that they represent transistors larger than the minimum size, then you can use them as you described.