If it is in the present tense then it would be compro
yo compro los zapatos en la zapatería
If it is the past tense then it would be compraba
yo compraba los zapatos en la zapatería
If it is the future tense then it would be compraré
yo compraré los zapatos en la zapatería
Honestly it could be any one of these but most likely the present tense form that is needed.