I think the following product could be very efficient for your analysis: CD31 monoclonal antibody (MA3100). You can access to the product by this link: https://www.thermofisher.com/antibody/product/CD31-Antibody-clone-HEC7-Monoclonal/MA3100
For characterization of HUVEC by CD31 immunofluorescence staining I would recommend using polyclonal (e.g. rabbit) anti-CD31 Ab with a FITC (or any other fluorochrome) labelled matching secondary Ab. Alternatively, you may use acetylated-LDL in live cell culture to check the purity/characterize your isolated HUVEC (if that's the reason you are using CD31immunofluorescence staining approach).