I have also used a subscription service like RealQuest. If you don't want to commit to paying the monthly fee for RealQuest you can also use a mailing list broker. Contact me for a current recommendation on what list broker I recommend when you mailing list are ready to purchase the list. Once you have your list, you need a service that will allow you to upload your list and the message that you want printed on your postcard.
This service should then print, address, and mail your postcards for you. Trust me when I say that you DO NOT want to do this yourself. It is mailing list cheap to have companies do this for you - but very time and labor intensive to do it manually. I have personally used the US Postal Service Click 2 Mail service and have been extremely happy with them. You should strongly consider using them.
I recommend you use a small 4 by 6 post card. You might see a bump in response from using a yellow card stock, which tends to cost a little mailing list more, but you will still get a good response from just using white with black text. You don't need to use fancy colors, photos, glossy paper or image advertising. I use a "typewriter" font message to my list of investors. What does my message say?