You can buy Sealcoat in smaller quantities.

Shellac has a unique quality with regard to compatibility with a wide range of finishes and makes an excellent sealer. It is commonly used to seal and provide a compatible coat between dissimilar finishes, hence the repurposing by Zinsser as a seal coat. Since it does not need to cure, you can recoat at any point in time, unlike polyurethanes. It is an easy finish to touch up if damaged. Other than table tops and bar tops, it's a great finish for furniture and my finish of choice for bookshelves and similar pieces. An acrylic finish isn't going to give you that nice honey color on tape that shellac will, especially not the water-based acrylics.