National Stadium next to MBK is one possibility. Another would be the big stadium at Ramkamhaeng University. I believe the Sports Ministry is located there as well. Getting close to the sideline shouldn't be too difficult, unless there's a popular foreign team involved.
I agree to you epixx. National Stadium is a great place to practice football. There are many teams playing in this stadium. I remember the time when our team played here. Everyone did their part to get a perfect practice.