Depending on its type ^{[e.g. aggregator, faucet and etc...]} and its status ^{[e.g. new or established]}, it differs from case to case, but the following thread might come in handy: List of Advertising Networks
^{Note: You can't view its content with an ad blocker and I prefer to not vouch!}

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For new crypto projects, I'd suggest directly selling ad spaces to other similar platforms instead ^{[most efficient route]}.