go to
http://www.farnell.com
- select country,
- search for "ptc resettable fuse",
- select holding current: 200 mA
- select tripping current: 300 mA
- use select
you should find something like:
http://nl.farnell.com/bourns/mf-r020/resettable-fuse-60v-0-2a/dp/1653473
or search with google for "PTC fuse" and find a store in your region.
maybe holding current can be lower as 200 mA, but tripping çurrent should be approximately 300 mA to protect your USB circuit on your motherboard.
Voltage should be > 5V so you could use 30V or 60V fuse.
If you can't find a PTC fuse, don't use it, use a USB power adapter as power source (about 7 euro). The PTC fuse is not necessary in the serial build, its there to protect your motherboard.