Fortunately, I didn't have to go far to buy spare tubes for my Marin Alpine Trail 29er (TweetyPot).
At one point in time, I had difficulty looking for a 29er tube. The well known bike shops I inquired from had all ran out of stocks. It's good thing a biker friend, Dhenz, was able to find a bike shop that still had 29er tubes.in its stocks. And, I bought all the remaining four (4) tubes.
Now, there's a bike shop in San Mateo, Rizal that sells 29er bikes and parts. So, on my way to Timberland this morning, I passed by the shop and bought two (2) Continental tubes. The cost is P250 per tube.