$4 for three sets, at Amazon
Your best bet is to try some tongue pads—thin felt patches that fill in the space between your foot and the shoe like a shim. They're ideal for shoes that feel a bit loose or if your heel is slipping (which can cause nasty blisters). The soft pads are about ¼" thick and self-adhesive. Stick one on the back of each shoe's tongue and it will push your foot back into the heel for a more snug, agreeable fit. They also add a bit of comfort, especially for those who wear their loafers without socks. You can likely find them at a local shoe shop or drug store, but there are also plenty available online.
If your shoe fits but your heel still slips or gives you blisters, look for smaller adhesive patches similar to tongue pads known as heel cushions or grippers.
$1, by Kiwi