I'm surprised this has only been mentioned twice, and not in much detail. (Maybe I've missed something).
Since I am building for telegram, I've been joining adult groups for some time now, and most of them get group lists posted (and sometimes deleted) automatically from Link Exchange Bots.
Some bots will require your channel to be over a certain number of users, while others are meant for small channels. Link Exchange is generally free, as long as the bot is allowed to post its list on your channel, but some will also have paid options, like listing you when you haven't reached the numbers they ask for yet, or displaying your link on a button on a post's inline keyboard.
The best way to find these bots might be to join related groups (you can find them through online directories) and well, just lurking around. Chances are they will be already participating on some link exchange. If your channel is too niche, you can still try in a broader but related category.