Other Rogue subclasses (Swashbuckler and Inquisitive) offer alternate ways to trigger sneak attack, but, since your build doesn't include them, you are stuck with the default ways.
Therefore, besides getting another melee combatant in your party, you are limited to ways of getting advantage on your attacks.
Warlock happens to offer a few ways to get that, though they require more levels:
First of all: True Strike cantrip, not the most optimal option (takes an action, requires concentration and works for only one turn), picking pact of the tome nets you few more cantrips from any class, so it might be a neat way to get some more utility stuff along the way.
Familiars can use the Help action to grant you advantage, and there are few ways to get a familiar, either a fancy one from pact of chain, or one from a ritual (by getting pact of the tome and relevant invocation or the ritual caster feat), though familiars are notoriously squishy and even the owl familiar (that doesn't provoke AoO with its flight) still can get shot down or get eradicated by AOE spells.
Another option is the ages old Devil Sight + darkness combo. But it'll annoy rest of the team and burn your spells on that.
Then there is the Shadow Blade spell, attacks made with it in appropriate light condition (I think it's shadowy illumination) have advantage (and the weapon created qualifies for SA).
That would be all I can think of at the moment.