Plenty of places, and it can be fairly cheap. Here's Ubiquiti's selection for example [0], the basic one is $45-50 [1] (and less in quantity). I don't recommend Ubiquiti stuff much anymore because the company has gone to shit overall sadly, but for specific dedicated application stuff they're still a worthwhile data point. GPON into an SFP is interesting too.
>And how do you clone the serial number of the ONU?
Actually getting support from the ISP for your own CPE would often be the stickier wicket I suspect. My fiber comes from a nice local ISP I've worked with for 20+ years now, I can get right to senior techs and they're happy to do whatever, and indeed are happy to use me as a guinea pig for trying out equipment. Getting the right OLT profile and auth info was just a matter of contacting them. Of course for that very reason it's less important since they're not messing with anyone's stuff anyway. Maybe the kind of ISP you'd most want your own full stack for in most instances is also the kind that'd make it harder/refuse? Though sometimes I've been surprised and with dedication/effort one can slip through the cracks or find an SMB angle.
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0: https://store.ui.com/search?mockup=gpon&q=gpon*
1: https://www.balticnetworks.com/ubiquiti-ufiber-loco-high-per...