Talk to me, people who have experience with buying shearling linings for your boots!

I have waited for the perfect pair of shearling-lined boots to come along, but often my favorite boots are not, or come with faux. Someone on here once said to just buy your own liners and put them in. Would you size up, in that case? I'm not sure if I should buy some liners first, so that I have them in the house when a pair of boots arrives to try on, or buy bigger boots first and then get the liners. A little chicken-or-egg problem.