Which should i use? lo/la/los/las Puedo encontrar si busco bien. Pienso -¿Dónde puedo encontrar___?
-¡Ya sé! Hay que buscar__ en

For the first three blanks, I would say use "lo" because, it means 'it', "i can find it if i search well. I think -- where can I find it?". If you just plug the words into the sentence it would all come together! For the last blank use 'la, los,or las" whichever floats your boat :) Explanation:

corras is the only one that is informal but doesn't follow the suffix rule. correr is the verb. corre is the formal verb form. corra would be used as the formal is corre ended with an a.

your best bet would be corras, it's the only one that is informal but is still wrong