The main reason that I shoot Leica is to use lenses from the last 80 years. I have lenses as old as 1931 that work with the Leica Digital cameras as they were designed- RF coupled and full-frame. Right now, the M9 has a 75 year old Zeiss Sonnar on it. For
film cameras, I tend to use my Leica mount lenses on a Canon P more than the M3. Picked up the Canon P in a trade for a lens that cost me $60 in parts, I made it into Leica mount. 45 minute job. That lens later sold for $350 on RFF when the new owner passed it on. With camera collecting, it used to be a relatively inexpensive hobby. Prices for vintage gear have skyrocketed, and 40 years of picking up stuff "cheap" offset the high cost of the M9. Picking up stuff that needs work, being able to do amateur repair, helps keep cost down. Some hobbies can be money-pits, others can be self-sustaining.
On the original question, looking for 39mm to 46mm Step-Up rings: A search of Ebay sellers turned up
Adorama in the US.
http://www.ebay.com/itm/Adorama-Ste...t=US_Filter_Rings_Holders&hash=item4cfb6e6964
Odd: I use a 40.5mm to 46mm Step-up, they are plentiful. And 39->49 are plentiful. 39->46 would seem like a popular item, I am surprised that S&W or another Chinese company does not make them. I use several S&W step-up rings for some odd-sized lenses, like a 40mm->43mm Step-up for the Canon RF lenses. These work well, and were ~$3 each shipped. The 39~46 from Adorama will run more, as the shipping is added in.