I believe there are many of them. Depends on the antobody you want to use.

Here are a few examples:

https://www.cytodiagnostics.com/store/pc/Gold-Conjugates-c161.htm

http://www.2spi.com/category/conjugated-colloidal/

http://nanocomposix.com/pages/bioready-nanoshells-nanospheres

Also, doing EM work; if you want to do miltiplexed detection, you may be interested in NPs with different compositions (for exemple Au-Ag alloy NPs) where you can use one antibody for each kind of NP composition. If this is something of potentiel interest to you, alloy NPs will soon be available from here:

http://www.photonnano.com/

You can contact them about such NPs.

You can easily make the conjugate yourself using the kit from:

www.nanodiscoveryinc.com