I have used AgTron before and found them to be highly reliable.  They are non pin type sensors so you don't get restriction issues.
http://www.agtron.com/PRODUCTS

I currently have Dickey John blockage sensors because they were the company option on the drill.  I've had a couple of sensor failures, but they do cost a lot less to replace than AgTron (although I never had an AgTron fail).  They are on the same monitor as the drill and don't require an extra monitor.  I had a separate monitor with the AgTron, but it was small and can be mounted out of the way since you don't watch it all the time.
http://www.dickey-john.com/products/agriculture/planting/

I'm sure there are other choices, but these both work well.