At 2 your son doesn't notice or even care what form your card takes. You can put stickers and a few words on an index card. They come in colors too.
I have noticed that there are cards that play songs or allow you to record your own message available at my local Fred Meyers. I was surprised at the number and variety. You might find one of those appropriate to send every once in awhile.
If you have a computer there are web sites that make it easy for you to make your own cards. Disney's site has locations for kids to design and print their own pages as well as pages to print out and color. You could do that and send them to your son by snail mail.
If you want "real" blank cards they're available at craft stores and in craft departments of stores such as FMeyer. All of the pages are blank. You could glue on pictures cut from magazines or even draw your own. This would take perhaps 5 minutes. They don't have to look professional and in fact will be more meaningful if they don't.
Another idea is to subscribe to a magazine geared to his age and have it addressed to him. Tell him that you are the one sending him the magazine. Perhaps have him bring it with him when he visits and the two of you read it together. Be sure to tell the magazine that it's to be addressed to him.
I haven't looked for them but when my daughter was in grade school one of the companies had a line of cards just for parents to give to kids. You might find the source on the Internet. However, at 2, your son isn't going to be able to comprehend most of those messages. Personal messages about what you feel about him, his interests, remembering activities together, etc. will be more meaningful. The messages in a commercial card will be more appropriate when he's older.
I am pleased to see a father maintaining contact with his son. He's fortunate to have a father such as you.