The only actual fruit of the four species set, the etrog is the most expensive of the lulav and etrog set. People sometimes pay up to hundreds of dollars for a single esrog. The citron is referred to as the “fruit of the goodly tree”. Actually, it is the only specie that is not mentioned explicitly by name in the Torah. The sages in the Talmud understood the passage as referring to the citron.
The etrog has both smell and taste thus symbolizing the righteous individual who possesses both good deeds and scholarship. The etrog grows in the shape of a heart, and therefore is required to be wholesome and unblemished like the heart should be.
This year buying an etrog from Israel is a bit complicated because of the laws of the Sabbatical year- shmita. However, it is not impossible to purchase a rabinically certified set, which can even be ordered online. So go ahead and buy lulav and atrog set online. Chag Sameach!