Look for Technics EPA-100 tonearm, It’s 10.5 inch tonearm, one of the best you can buy for your SP-15, they are often available on ebay (in perfect condition) from the Japanese sellers.
Tonearm for Technics SP-15
I have a Technics SP-15 turntable with a Grace 707 arm and a Grace F9E cartridge with a Soundsmith new stylus. This combination sounds great but is limiting. The 707 tonearm has a fixed headshell and it does not allow me to easily swap other MM high compliance catridges or match compliance requirments MC cartridges. Suggestions welcome from those more expert than I.