Here is an Agamic mantra (or Tantric mantra) dedicated to Lord Indra taken from the Mantradeva Prakashika.
Im IndraYa Hrith | asya Brahmarishi Pangtischandaha Indro DevatA Im
Bijam Ayethi Shaktihi |
Im Indraya namaha, for this mantra the Rishi is BrahmA, Chandah is
Pangti, Devata is Indra, Bija is Im, and Aya is the Shakti.
Rishi Nyasa and KarAnga Nysas are done like - Sirasshi Brahmane
Rishayae namha...., Im Angushthabhyam namaha, Im Tarjanibhyam SwahA,
... Im ShikhAyai Vaushat, Im KavachAya Hum etc.
Dhyanam for Lord Indra for this particular mantra is to be done like this:
Pitavarnam sahasrAksham Vajrapadmakaram Vibhum | Sarvalankarasamyuktam
Naumindram Dikpatiswaram ....||
Indra Deva is having a yellowish body. He has thousand eyes. His one
hand holds his weapon, the vajra and in his other hand he holds a lotus.
Prostration to that Indra who is decked with varied and many ornaments
and who is the lord of directions.
Next to be done is the Shadanga Puja which i am not giving the details of. Ayudha Puja (i.e Puja of the Vajra) is also to be done.
The purascharana of this mantra is done upon completing 1 lakh japa and doing 10 thousand homa with ghee mixed sesame seeds.
Asya Purascharanam Lakshyajapam, Ajyena Tilairayuhomoho
Chatushkonasthapadme Navavastraveshtitam SthApayitvA Gandhodakena
Sampurya Tatra Tatra SaparivAramindramAradhya Sahsram JaptA
Tajjalobhishekam Brastarajasya RAjyaprAptiranyeshAm Parama
In a square mandala an eight-petaled lotus is to be drawn. On that
lotus a kalash (pot) decked with the nine jewels is to be placed. The
kalash is then filled with gandha mixed water (sandal paste mixed with
water). Thereafter, Indra has to be invoked, along with his family, in
that pot and worshiped by doing a thousand japa of the mantra. Now the
devotee should bath in that water. This is called IndrAbhisheka. The
person on whom such an abhisheka is performed can even get back his
lost kingdom and gets many other priced belongings and etc.
Also following is the gayatri mantra for Lord Indra as given in the Linga Purana.
Devaraajaaya vidmahey vajrahastaaya dhimahi, Tannah Shakrah