Setup your own (small / tiny) Certificate Authority (CA)

For more than 100 certificates, you could use: NewPKI, OpenCA, IDX PKI which
are opensource projects.

A smaller solution is TinyCA (http://tinyca.sm-zone.net/). Note that this is just a GUI front-end to openssl.

From Wikipedia:

EJBCA (java-based)
OpenCA (the most popular one)
OpenSSL, which is really an SSL/TLS library, but comes with tools allowing its use as a simple certificate authority.
gnoMint
DogTag (This looks really good)
XCA