The original text of Bhagavad Gita containing 700 shlokas verses in Sanskrit along with English transliteration is available with Gita press, Gorakhpur, India with prices ranging from $.10 to $1. All the publications from Gita press are highly subsidized.
Relating to Bhagavad Gita and Upanishads (independent spiritual treatises) there was nothing better than publications from Gita press, a storehouse of knowledge.
Gita press Gorakhpur has presence in main cities of India and also online. I recommend if possible going through all their publications as they have different editions and choosing what best suits our appetite based on our thinking pattern.
Commentary on Bhagavad Gita in English and Hindi is available from Gita press from renowned writers like Jayadayal Goyandka, Swami Ramsukhdas and Hanuman Prasad Poddar. Their commentaries are par excellence, the best available in the world.
Furthermore, commentary from Adi Shankaracharya in English is also available. It need not be mentioned that Adi Shankaracharya gained enlightenment in his lifetime.
When referring to Bhagavad Gita and Upanishads, one must never indulge in any other commentary other than those from Gita press Gorakhpur.
The publications from Gita press are not only highly subsidized but they are truly serving mankind at their level best. I personally advocate for whomever possible making large donations to them.