Try "boat motor recyclers" they are in Burnsville MN. If they don't have the parts, they will put it on their search list! I too have a '73 Johnson 135hp, I purchased a complete system from them 2 years ago. These parts are getting hard to find, so you might have to buy it piece by piece. There is several listings on ebay for the main support plate. Keep this in mind: the '73 135 did not come with PT&T...the 135 has the bolt pattern to accept the main support plate, it'll bolt right on, but the 135 never came with PT&T. So, when searching, don't use the term '73 135hp, PT&T started in '74.
If you get lucky and find the main parts, Sea-Way Marine in Washington State should have most of the smaller rebuild parts if needed, bushings, seal kits etc. I am very familiar with that PT&T system, so if you have any questions, just message me.
There is a few special tools required to rebuild Rams and such, I have those tools...here to help any way I can