1. Justin.TV: Problem is that it isn't always available for stream and PQ not great.
2. Find a DVR solution like TiVo, Simple.TV, HDHomeRun, EyeTV, ChannelMaster DVR+ and record from CBS and syndication
3. Go to your local library and check out the DVD's free. My library lets me check out 3 DVD's at a time so. Maybe your town as well. Plenty episodes per DVD.
4. Through PLEX and PLAY**. Check both supported and unsupported channel scripts.
5. Try "AirPlay/Casting" Roku apps and see which ones allow you to cast from your smartphone to the Roku: Twonky, PlayTo, TrimePlay, AllCast.
6. Available on Amazon (but not sure if on Prime subscription as I don't subscribe to AP)
If I missed anything, feel free to add other methods.
Good luck and let other TBBT fans know how you are able to watch it FREE on Roku.
I DVR them using EYETV on my Mac Mini.
2xRoku2 XS,Roku1+SE,Logitech Revue GoogleTV,Raspberry Pi+XBMC,Mac Mini+EyeTV DVR+RabbitTV+PLEX+Kodi,TiVo Premiere,Sony BluRay+streaming apps,Equiso Android TV,FireTV,Nexus player,Simple.TV Local TV:QAM from ISP coax line. PLEX+unsupported apps