I am planning on doing an offroad bike packing trip around Europe. I have acquired a Garmin Etrex Venture HC to aid in my navigation. I plan to use it to assist in navigation but will not be reliant on it.
Does anyone have suggestions on where to acquire either free or cheap topo maps that I can upload?

openvttmap for mtb, velomap.org for on road. But… don’t just do it the day before and expect to know how it works and how to use it. Download the maps for where you live and try it out first. There is various tweaking to be done to get the best out of it, especially if you are going to use the routing. Note that these are crowdsourced maps so generally aren’t as good or accurate as the official ones but my experience is that they are usually great and up to date. If I were you I would also download them for the area you are going to and compare them with an online map to ensure they have proper coverage where you are going

As above. I have used the velomap and openvttmap coverage for most of Europe and find it to be great. There are small issues at times, but if you can generally read/understand maps & terrain they are fine. If I am going to a area where I know I will spend some time I will also download the aerial images at a fine scale to cover any complex bits. Not sure if you can do that with a Garmin, but certainly works well on a smart phone

Windows/Linux – OpenMtbMap Map Download

got UK and france on my Etrex

http://garmin.openstreetmap.nl/

Anyone know if you can change the language for the individual downloads? Got Bulgaria but, well, can’t speak a word..

http://wiki.openstreetmap.org/wiki/OSM_Map_On_Garmin/Download

http://www.velomap.org/