I own a Minosaka Jidai Koshirae Iaito that needed a new Mekugi after nearly 10 years of (beginner-)training. Since it's very hard to find spare parts outside of Japan, I'm glad to have found Seido Shop selling Mekugi, which come from Minosaka itself. The customer service of Seido also helped me with my order and enabled me to order, even though it was below the minimum order price.
The Mekugi themselves are in the usual high quality of Minosaka and therefore fit perfectly into my Minosaka Iaito! But there is also the possibility of making Mekugi in other dimensions, if the Iaito is not from Minosaka. If you have a black Tsukasame you have to blacken the ends of the Mekugi by yourself, but probably everyone should be able do it with a black pencil.
Since the set consists of two Mekugi, I hope to train for many more years without any problems now.