Hellish Rebuke as Paladin?

I play a tiefling paladin. Have Hellish rebuke, love it, how do I get it as an actual multiple per day thing (Currently 1/day as tiefling.)

I take it you're playing with 2014 rules?  Some GMs will allow characters to use regular spell slots for their innate spells (since later books like Mordenkeinen's Monsters of the Multiverse did that with all its PC options), otherwise it looks like you're going to need to multiclass.

Oathbreaker Paladins also get Hellish Rebuke as part of their Oathbreaker Spells feature.

If you are playing under 2014 rules,  you could take the Magic Initiate feat and choose Warlock as the spell list from which you learn your chosen spells. 

Otherwise, you will need to take at least 1 level in Warlock, as I believe those are the only ways to get Hellish Rebuke.

Magic Initiate under 2014 rules only lets you cast the spell once per day.