sells singles, so you can build any set you want.
You'll need a relatively longer string to go string through on your Lakland, something wound to 38" is ideal, I use a custom set of D'Addario Super Long XL's from Bass Strings Online. Other strings that are long enough and available as custom sets would be DR and Ken Smith. He may have more, send him an email and see. He's also a member here, and you'll get a TB user discount!
Another option is just buying a 4 string set and adding the single C string. That may open you up to more string options since the C string is not tapered at the tuner end so it doesn't really matter how long it is.
Here's a direct link to the singles: http://www.bassstringsonline.com/Sin...ings_c_53.html