K.B.
Here are a few sites with the infant seat belt/harness on it...
http://www.inventiveparent.com/cgi-bin/shopper.cgi?preadd...
http://www.amazon.com/s/?ie=UTF8&keywords=cares+harne...
I flew alone with my daughter a few years ago (she was 4.5 months) and she was on my lap but I did not use/know about the flight harness so I do not have any experience with them.
I will be flying again by myself middle of April, my daughter is now 2.5 years old, and we got her own seat. I will be bring the carseat along, there are a few products out there to make it easier to lug it through the airport...
http://www.amazon.com/s/?ie=UTF8&keywords=tote+a+tot&...
http://shop.happymothers.com/Tote%20A%20Tot%20Kids/Car%20...
I am looking at tote a tot that cost $25 since it has extra pockets.
When you are landing and taking off have your child drinking or eating/chewing something, that helps relieve the ear pressure so hopefully not upset about the "painful" change in air pressure.