www.superbrightleds.com has a good selection at reasonable amounts.
They say to make sure you match the LED color to the color of the lens for best visibility.
I have not ordered from them before, but looking at their website I found the following...
1156
Red 30 LED Part #1156-R30-WV $8.99ea
Amber 30 LED Part #1156-A30-WV $8.99ea (currently out of stock)
1157
Red 30 LED Part #1157-R30-WV $8.99ea
Amber 30 LED Part #1157-A30-WV $8.99ea
(shipping is only $2.99 for 1st class and $5.45 for Priority if the order is <$100)
Also keep in mind that if you change your turn signals to LEDs you will want to get a different flasher relay to make them blink at the correct rate.