I seem to recall a thread a long time ago regarding whether you could change out the windows on an exterior door. The answer seemed to be that it would have to be custom work done by a carpenter.

If your door has glass in it now, you can put in replacement glass yourself. The insert is held in with molding attached with screws. Here's a how to I found.

We considered doing something similar, but our door is already half glass. I found insulated replacement leaded glass inserts at the big box store but it was fairly pricey (about the same cost as a new door) so we decided to make a wood grille instead.

I found a blog where someone had a full glass insert put into their whole wood door for under $100. I'm thinking the $100 was just labor and they didn't divulge the cost of the glass. When I called a glass shop, just replacing it with the same clear insulated glass we have now was over $150 for the cost of the glass without installation.

I don't have a habitat store or a salvage store near me and didn't check any surplus stores.