In preparation for our Mountmellick embroidery stitch along, starting in about a week, I thought I might direct you back to an old post on Mountmellick thread. Mountmellick thread IS different to work with compared to other thread, and the post explains a bit about it.
White Threads is the blog of Yvette Stanton, the author, designer, publisher behind Vetty Creations' quality needlework books and embroidery products.

I’ve found a bamboo crochet thread, No. 10, that is very white and soft. How would the No. 10 size compare to the weights of the Mountmellick threads? I’m wondering if it would be an adequate substitution. It is easily available and quite inexpensive.
Hi Terry,
I do not know what size No 10 is and how it would compare with Mountmellick thread, so I cannot advise you on that. However, all the bamboo threads that I have seen have quite a sheen, sometimes even comparable to silk. As Mountmellick thread should be completely matt – with NO shine – it seems unlikely to me, based on what I have seen of bamboo threads, that it would be suitable. Not having seen the actual thread you are talking about though, I cannot really say.
As Mountmellick threads are quite readily available through our website and also Tanja Berlin, I would suggest that you could try some of the real thing before looking for substitutes.