One of the most frequent questions I get is "where can I find the threads you use for buttonholes?". My go-to gimp is Agreman and the silk twist I usually use is R753, both products by Gutermann. It has sometimes been challenging to find it at retail, especially in the U.S., but there is now a new source over here. I haven't used them- the National Sales Manager at Gutermann is a friend and she let me know about them.
Bay Tailor Supply is now stocking these items. If someone orders from them, do let me know how the service is.