one can only come across such things serendipitously
Definitions (粵典–words.hk)
語句
好難得嘅嘢,強求都冇用嘅,要咁啱遇到先得 something so rare that can only be encountered by chance, and not through active search
今次嘅機會好難得㗎,真係可遇不可求,你要好好把握啊。
今次嘅机会好难得㗎,真系可遇不可求,你要好好把握啊。
gam1 ci3 ge3 gei1 wui6 hou2 naan4 dak1 gaa3, zan1 hai6 ho2 jyu6 bat1 ho2 kau4, nei5 jiu3 hou2 hou2 baa2 aak1 aa3. This opportunity is very rare that you won't come across through seeking. You have to seize it well.
Definitions (Wiktionary)
phrase, of something good or fortunate
can only be come across serendipitously; good things may happen, but may not be sought; just have to let it happen, you can't force it