I need to locate the vacuum pump/valve to check for leaks on my 2000 Ford Escort?

My car is hissing very loudly and won't pick up speed past 3000 rpm. We have changed the spark plugs and leads. We also changed the air filter and used a carb spray.
I have identified that it may be a vacuum leak as we have tried all other options.
Please help. I can't afford to take it to a garage. Helpless female :)