We get all sewing done at Mercado 2 in Surquillo. When you go in the main entrance, in the first couple of rows there are several seamstress shops. We don't have one in particular, we just pick one that doesn't look busy at the moment. We've done everything from hems to repairs to completely tailoring uniforms for the boys for school.
Eddy gets all his pants "fixed" at a little place just a few blocks from your apartment. He normally charges 5 soles. It is just a "hole in the wall" but it is the father of one of our friends. For the price we have always been satisfied. Let me know if you want the address of give Eddy a call and I am sure he can help you out.
I don't know where you're located but in Miraflores, in the little computer market located on the left hand side of Porta halfway between Benavides and Schell, or across Diagonal from the new Wong if you're coming from the other side of Diagonal, there are 3 or 4 seamstress / tailor shops that do alterations. I had several pairs of pants hemmed just last week for S/10 apiece, same day service.
On the corner of Larco and Benavides there is a casino, Atlantic City. Directly behind this casino on Bolivar Street there is a sasteria with several employees who will do whatever you need to your clothes. I dont remember the prices. It is on Bolivar street between larco and alcanfores.