Center support brackets come with all the blinds that require one. Having personally installed several hundred thousand apartment blinds of the vinyl type, what I have always used is a # 6 hex headed screw that is 2 1/2" long. If you will put it through the right hand side of the slot that is in the middle of the headrail it will prevent the sagging and keep tenants from removing the blinds. The slot in the middle of the headrail is intended for the center support bracket, which I personally never used as it greatly increased my install time. The screws we're much easier and faster.
If this will do it for you, great !! If not, the center supports will cost .25 cents each plus shipping. Screws as described above are about .05 cents each at your local lumber/hardware store.