check out bridal section of Michael's or Hobby Lobby. They usually sell strings of white lights.

I get mine from Boleks crafts. They have great customer service and fast shipping.

Here is a link that might be useful: lights

I have some that I would be willing to part with. I'm not sure how many I have but if you would email me I could let you know.
psperkins@i2k.com

Please put THS LIGHTS in the subject line.
Mine also have an on/off switch on them that I put on.

There are a couple places you might try.
One is National Artcraft in Ohio. I believe the site is www.nationalartcraft.com or you might try Lamp Specialties in NJ. These are wholesalers and they have these items all the time. I don't remember the url for Lamp Specialties so you might try a Google.
We used to buy the lights year round from these two companies.
They're basically caramics suppliers but you can get a lot of other things from them too.

If you are looking for the one bulb with a clip to put into the hole drilled area, I get mine at the hardware store locally. I found them at our Menards store but I'm sure Lowe's and Home Depot should also carry them year round.

I get the single bulb ones at Hobby Lobby. They are in the miniature section. I wait until they have a 50% off sale and buy them for $1.50 each.