How do I get LED bulbs to work correctly in my car?

have a 2003 Ford Crown Vic Police. I already know that without the resistor the turn signals will flash with bulb out indicator. My issue is the front turn signal bulbs. The LEDs will not come on when switched on. They only work for the turn signal. The regular bulbs (3457/3357) work as they should and stay on when switched on. How do I fix this problem? I can't find 3457 and 3357 bulbs in LEDs. I was told that 3157 is the LED sub.