"weaponized" ones are actually not that deadly.
Modified however you basically have AT weapons right there.
Most firework rockets you can buy use a rather weak rocket motor, but it's more than enough to push a grenade beyond throwing distance.
Take a cardboard tube, stick a handgrenade (or a firecracker in a metal tube) to one end, and the firework engine to the other end. Light both fuses, ensuring that the grenade/firecracker fuse is longer than the motor one and watch the rocket you just built fly towards the SWAT car. For extra accuracy launch it from a tube, and add springsteel fins, like the OG Panzerfaust had.
You can probably get some tannerite and use that to build an impact fuse for the warhead.
Igniting the rocket engine would be a little more difficult, but a blank cartridge fired with a normal trigger/hammer setup should work as well.
Honestly, there is nothing you can't build yourself with the required amount of effort.