You can look for the drivers needed for the onboard cards straight away and just inject the installation media with the correct driver using image builder, see below for a how-to.
Using the vSphere ESXi Image Builder CLI | VMware vSphere Blog - VMware Blogs
If you want a cheap option and got the right PCI slots (there are 2 flavours) the following might be a nice option to get:
http://thehomeserverblog.com/esxi/intel-pro1000-dual-gigabit-nic-pci-x-card-in-pci-slot/