This one has preloaded custom vehicles.
http://www.vehiclesforgarminnuvi.com

this one allows you to upload a SRF file and it returns a PNG file that you can edit with GIMP and then upload the PNG and it returns the SRF file for you. (MS Paint doesn't work)
http://techmods.net/nuvi/

I used it to create my pajero.
I only edited the first top and bottom image of the PNG file as I only use the track up feature on my nuvi.
