The usual suspects like a 10 1/4" blade with a Ø5/8" bore (sometimes with a diamond knockout) or a 16 5/16" blade with a Ø1" bore aren't a problem as they're still available at a reasonable cost new. I have an old saw that needs a 9" blade to be mounted on a 1" square arbor and I have no saw shops around that modify saw plate bores. There's a sharpener not too far from me that uses CNC equipment and do an excellent job for what they charge but sharpening is pretty much it. I'm shopping for a Skil 127 which takes a 12" blade with a diamond knockout and those seem to be very rare also. I have a Skil 117 groover that needs a 7" stacked dado set, 2" wide to fit on the Ø3/4" arbor and Forrest can make one but not as a cost effective option.
You can try a machine shop. Maybe have the square arbor turned into a round one. Stuff like that.
my 2 cents
find a 10" blade, buy 10
have them modified to the square arbor
modify the machine to use the 10" blade
or bite the bullet and get a 10" machine
I've done a few blades with my Bridgeport and rotary table. It takes more time to get the rotary table in position than it takes to do the actual work. My local saw sharpening shop has a Bridgeport they keep sat up just for that task. I try to get folks to go to them as they do good work and since the machine is ready to go they are also cheaper. Think they charged $20 a few years ago.
Sell to a hobbyist that will not need more tooling and use the proceeds to buy something that is supported for a daily driver? The phrase i remember "It isn't about the gun, look at the nails"
This (these actually) are restorations save for the Skil 117 groover, they won't be daily drivers but I'm not object to mild modifications to allow use of more readily available, and often much less expensive tooling. I posed this question a few places and I've already got some ideas as to who's done this more than a few times and know what does and doesn't work. If I did need something to put to work every day, there's several overseas colors and flavors coming from economies I'm disinterested in supporting, however that's not the case.
After posting I thought "I am the guy that twice this year has been to the deep bolts on a daily driver '75 Zettlemeyer ZL500 that even the factory has said, don't be sending that junk back here :D"
I think the local electric motor shop could turn the arbor down.
Do you have an old blade? 9" blades are not exactly in short supply. The trouble is the square arbor. If you have an old blade that you can use to scribe the arbor shape onto the new blade by, there are multiple ways to do it, and I would even try it once with a die grinder.
I'd like to see a picture of whatever this saw is.
:P