Forget replacing the subframe unless it has really rusted through... they are very hard to find. It is a common fault but most scrapyards don't save the subframes when they break a car.
You have to do a proper repair... as was said above get it inside a garage and up in the air, grind and rub away all the rust then treat with a rust-killer / converter solution (Hammerite do small jars), then paint with 2 thick coats of Hammerite paint.
If you can spray someWaxoyl or similar into the subframe from above this will help to preserve it also.