For the most part, I believe it is all about the management and troubleshooting ability from the ISP side. You can buy ONT devices, but I think that the OLT device of the ISP must support the one you bought, which really defeats the purpose of buying it. There are even SFP modules that can work as an ONT interface.
Really your best bet is to ask your ISP if they can bridge a port on the ONT so you can use your own router after that without double NAT. In this scenario the ONT will be functioning mainly as a media converter.