I use a metal strainer on sauces from the supermarket. I've also made my own - there is a simple trick to removing the seeds when you make sauce:
1. Cut a small "X" in the bottom of a tomato
2. Put the tomato into boiling water for 30 seconds
3. Drop immediately into ice water to stop any cooking until cook
4. Peel the tomato from the "X"
5. Cut in half and dig out the seeds with your finger (rinse with cold water)
Best of luck!