I would recommend a good shop to fit them, in part because they do have a variety of different volumes available. Tighter fitting shells may take only a plug liner, while the luxury liner is available in a low, medium and high volume size. You would want to find a shop that has a large selection of liners to get the exact fit.
If you have a shop that only stocks a few liners, or tries to do a one size fits all sale; or if you don't have a shop nearby, you can order direct. Email them and they can help walk you through choosing the right liner. You can trade out or return as needed until you get the right liner, but you have to pay the shipping. You can heat the liners and fit them at home if needed. Intuition recommends the blower type of heat over the older oven style. You can also heat them by microwaving baggies of rice to 120oC and placing them in the boot.
Several of the boot manufacturing companies are opting to have their stocks liners made by intuition instead of making their own now; the materials and workmanship of the intuition is the highest quality available.