I don't know if there are Thompson calfskins. But Scripture Truth book company usually has good prices on Thompsons.

You can look here:
Scripture Truth - Christian books and study materials.

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Fredrick T. Greco
Senior Pastor, Christ Church PCA (Katy, TX)
Christ Church Blog

"The heart is the main thing in true religion...It is the hinge and turning-point in the condition of man's soul. If the heart is alive to God and quickened by the Spirit, the man is a living Christian. If the heart is dead and has not the Spirit, the man is dead before God." (J.C. Ryle)