Most products, as you can imagine, are illegal.  The difficult part is deciding which product will work for you.  The truth about supplement is that they do not typically add to your natural performance.  They make up for what you are lacking in your training nutrition and routine.  Some offer a more intense workout, others protien or carbs.  With a great nutrition plan the body does pretty well for itself.  That being said, what are you lacking.  Are you taking in a balanced diet?  A lack of just 5% water will decrease performance.  Are you getting the right nutrition before races?  Blood sugar will be very key in an event like the 400 meters. 

The reality is, if you are carrying bodyfat and you are feeling a drop in energy around 250 meters, you can point to a couple of issues.  Insufficient nutrition (lack of something, excess of something else)or inefficient training (too many long runs, not enough conditioning, not enough rest between training days) will cause even the best runners to have a bad day.  Spending money on supplements when you can see the same results by training smart seems a little silly.  Besides, eventually you will reach your near full potential and need the supplementation.  

If we were near max potental and we wanted to be more competitive I may suggest a Creatine Supplement (depending on the rules of the league/club) or maybe a nitric oxide supplement depending on the client and the rules.  Most likley the performance can be improved with a peanut butter sandwich and a glass of chocolate milk 15 min before warmups.