Visit a nearby music equipment store and buy tweeters of your choice. You will find a huge variety of tweeters available in the market, depending on the purpose and style. Cone, dome, piezo, ribbon, horn, plasma etc. are some of the most common types of tweeters available in the market.
Tweeters that cost more usually come with crossovers. You can check it in the specifications of the tweeter. If the tweeter you bought does not have crossover, then you will have to buy one that complements with your tweeter. You can ask the store keeper to help you out in that regard.
In addition to that, you should also buy a tweeter that complements with the power output of your sound system.
For instance, if you buy a tweeter that has a very low power compared to your sound system, then it is most likely to explode when you will turn the volume high. You can tell the store keeper about your sound system and the size of your room, in order to get a better advice when buying a perfect tweeter.
Check the wiring of your sound system. Point out the wires that connect the main system with the speakers. If you have different sized speakers attached to the amplifier then you should know about the wires that connect to the smaller speakers and the ones which connect to the bigger one.
Take out the wires of the tweeters and connect them with the smaller speakers. If there is no option to connect them to the smaller speakers, then you can check your amplifier for any extra jacks for the audio output. Connect the tweeters to the output jacks present at the back of the amplifier. If you cannot find multiple output jacks, then you can buy a converter from the market, which will convert one output jack into multiple jacks.