You can always try radioshack.
I put both the 1k ohm pot and the rheostat because you can choose either one. The rheostat will let you control the current going to the diode with more accuracy, but it's a big a$$ thing. And the 1k ohm pot is small but it's a little hard to adjust.
Total of parts with the rheostat is $11.44
Total with the 1k ohm pot: $10.14