Looking for app to fill credit card info on form after swipe from a card reader
They clearly exist, but not necessarily free; I googled a bit and found 

http://idtechproducts.com/component/content/article/438-mobile-readers/125-imag-pro-mobile-magstripe-reader.html

Assuming you already have a card reader, have you tried contacting its manufacturer to see if they have any recommendations?  Most iOS app/reader combos will be full POS or transactional systems, like Square.