Can anybody point me to a site where to buy 50g 20"x1.35 tubes?

My Swift is nearing completion, I am still waiting for a few items to come in the mail. Currently it is 9.5kg. I rode it to work today an all I can say is wow! it is fast. I have a 58T ring in the front, it is blazingly obvious I will need the 60T one since I am riding much faster.

Once I have done everything on the plan, it may squeeze in below 9kg.

Photos and full description/specs to follow on completion.