There is a Japanese company called Anex that makes short ones. eBay would probably be the easiest to buy some.
I purchased after Christmas from a South Korean eBay member "allaview8979" Anex Phillips and slotted mixed short bit set; and a hex bit set from his store here:
LINK. My first shipment which included two Anex No. 62 replaceable bit handles and an Anex NO.6103-F Ultra Low Profile Slim Offset Hex Wrench Set took 2 weeks to arrive to my home. He charged me only $8.00 for shipping.
My next package (ordered a couple days before New Years) included the Anex AK-51P-B8H Ultra-Short Hex Bit Set. He charged me $5.00 for shipping. It took a month to arrive to my home.
I was very satisfied with his communication via eBay. The Anex short bits look high quality; have a groove below the bit for easy removal by hand or a plier; and are each 19mm in height. Regular screwdriver bits are 25mm in height.
By the way, he offers a large selection of Olfa (made in Japan) cutters in his store here:
LINK.
Dave