Hard to find one, some sellers sale silicon case for 645 but actually for 245. I've tried one and it fit 90% with 645
m.lightinthebox.com/.../cases-for-forerunner-645-forerunner-645-music-tpu-compatibility-garmin_p7964207.html
www.amazon.com/.../ref=cm_sw_r_cp_apa_glt_fabc_JG3CWKCBB1FN5M6B3BZE
m.lightinthebox.com/.../cases-for-forerunner-645-forerunner-645-music-silicone-compatibility-garmin_p7973743.html