There are many Linear Technology boost regulators furnished with LTSpice.
You can use the Parametric Search on their website to find a suitable regulator.
If you don't find one to your liking, or you must use a regulator from another company, you will either have to make a PSpice model or download one, then make a symbol for it, put them in the proper subdirectories, and then restart LTSpice.
You did not say which boost regulator you wanted to use.