These are very common in Japan because of the great importance placed on good penmanship. I do so wish more importance was placed on penmanship in the West as well. As a teacher I cannot describe the handwriting I see today. Bad handwriting is not as problematic for me as people who, quite simply, cannot write. They are so out of practice that they battle to put pen to paper.
In Japan, Shitajiki – also called pencil boards or writing mats – are very common. They give a nice firm surface on which to write and are also invaluable when writing on thin paper (such as Tomoe River paper) as they prevent pressure marks on the pages underneath from your pen. This is fantastic for lefties like myself who hold their pens more tightly and press harder on paper than righties. This is largely from the way we were taught to write.
In Japan the most common size is B5 which is bigger than A5 and smaller than A4. So most Shitajiki are B5. Though you can find A5 ones on the Nanami Paper site (look for ‘writing mat’), and A6 ones on the Hobonichi site (look for ‘pencil board’). You can also find A5 ones on eBay if you look for ‘mini’ or check the size specifications in the description. A5 size is 210mm x 148mm (21cm x 14.8cm). You can also often choose between hard and soft, and matte and smooth. I like soft smooth as it has give and does not impede my writing.
I prefer clear ones but my favorite is the one above. This type of Shitajiki is often printed with artwork, promotional slogans and anime/manga. They generally are very collectible as they are done in single runs. I get mine from eBay seller tsumujikaze-hsik as they are cheap, quick and have many to choose from in A5 size. Make sure of the size before you buy.