What you can redownload for free from the store will show in the Purchased link under Quick Links on the right-hand side of the iTunes store homepage on your computer's iTunes, and the Purchased tab in the iTunes Store app on your phone - if the versions of the singles that you bought have been removed and replaced with new versions then they won't show, they would be treated as separate purchases.
Or, and if you still have them on it, you can connect your phone to your computer's iTunes and do File > Devices > Transfer Purchases to copy them over to your computer so that you add them to your backup of your downloads/library.