Yes, you can buy half a Bitcoin. There are currently 18.5 million Bitcoins in circulation, so there are plenty to go around. You can buy half a Bitcoin by using a Bitcoin exchange or broker that allows you to fractionalize your purchase.
It is not possible to purchase a half Bitcoin. Bitcoin is divisible up to 8 decimal places, meaning you can purchase a tiny fraction of a Bitcoin (0.00000001 BTC). However, it is not possible to purchase half of a whole Bitcoin. Be aware that platforms offering the ability to buy half of a Bitcoin may be fraudulent and should be avoided.
For example, if you wanted to buy half a Bitcoin for $500, you would need to find an exchange or broker that allowed you to do so. Once you found an exchange or broker that allowed fractionalized purchases, you would then need to create an account and deposit your funds. Once your funds were deposited, you could then place an order to buy half a Bitcoin. It is important to note that not all exchanges and brokers allow fractionalized purchases, so be sure to do your research before committing to a purchase.