Dear Afra,

for immunostaining you can use von Wildebrand factor or CD31 (PECAM) performing standard immunostaining protocol:

fixation 4% PFA, permealization in 0.3%triton, blocking and Primary and secondary antibody.

  Good luck

  Cesare

I agree with the VWF staining.  Here's a link to a vendor:

http://www.abcam.com/index.html?pageconfig=resource&rid=10094