Realistically speaking, since the first country to recognise and create diplomatic ties with Soviet Russia was that same Estonia in 1920 after signing the Tartu peace treaty, it is incredibly unlikely that they would've found any foreign allies at all.

There was no way any actual country with any actual amount of power would've recognised, much less helped a communist country back then. It would've gone against the survival instinct of all the rest of the world. Which we see in that fact that the British, USA and Japan all invaded Soviet Russia in the attempt to overthrow the communist government.

The only option that also proved to work in the end, was the application of military power itself. Soviet Russia could've focused its military power towards Estonia in late 1918/early 1919 instead of turning focus inwards - towards fighting a civil war within Russia itself. However it wasn't just about ill-allocated resources, that stopped Soviet Russia from conquering Estonia, but rather the order of priorities. Soviet Russia was incapable of fighting every single enemy at the same time, and the forces within Russian core territory marching towards Moscow were infinitely more important than sending a conquering army against people that weren't too enthusiastic about Soviet rule. Similar to how Soviet Russia didn't (initially) try to stop Finland from secceeding from Russia.

To answer you actual question: the only option would've been to direct further forces towards Estonia in late 1918/early 1919 from other fronts to crush the nationalist governments. This would have to come at the expense of some other front. Choose your liking.