Ecards are appropriate for many holidays and special occasions because even if you do something as an entire staff, for example for National Boss Day, an ecard gives you the opportunity to express your personal appreciation and prayers. Following are some sources for ecards, not only for this holiday, but for anytime. You don't always have time to create what you want to communicate, so use the free resources below.
Christian ecards for National Boss' Day
Dayspring
These are not specifically for Boss Day, but for the workplace in general. They have a Christian message and could be encouraging to send to fellow church workers any time of year. While you are at this site, look at some of their cards for encouragement—just to encourage you.
Free ecards for Happy Boss Day
Punchbowl
This site has a really cute way they send the ecards in a digital envelope and how it opens when it is received. They also sells gift cards that you can send with the ecard and has offers for other gifts. Could be a handy thing if you wanted to buy something, and if not, just click through.
http://www.punchbowl.com/ecards/boss-day
123 Greetings
The site here is a secular site, but they have a HUGE variety for men and women bosses, plus fun cards that dance and play music.