You can make springs from round wire spring stock.
Use pliers with rounded jaws to prevent nicking or scarring the spring, which causes them to stress break.
You can also make a spring bender device by putting several nails in a wood block.
You can buy spring wire stock from Brownell's:
You can also buy a new AMT spring for $4.25 plus shipping from Gun parts Corp:
Another possible source is Jack First Gun Parts: