I normally plant bulbs in the fall however I was just in Amsterdam and the tulip bulbs were so cheap I couldn't resist!
Should I plant them when I get home or store them (if so how) until October?
Just store them in a cool dry place until the normal planting time.
You could plant them now. They won't start growing at the wrong time of year (the bulbs can be left in the ground all the year round which is how they grow in the wild of course), but you have a slightly increased risk of disease or of something eating them before next year - or simply forgetting where you planted them and planting something else in the same place!