Every bike shop and on line bike supplier in the world will sell that size tube. I buy two sizes: 700: 18-25; and 700: 28-35. Wider tires get the larger size.
I buy my tubes from pricepoint.com, but that really only makes sense if you are buying in bulk, otherwise the shipping will get you.
Tubes are a pass/fail kind of thing to me. If they hold air, good, if not, no good. So until they fail, I do not replace. Now old tires start to crack. I typically toss old tires a lot sooner than old tubes. On old bikes I pick up, 90%+ get new tires, 20% or less get new tubes. Most get new rim tape.
On smaller internet orders, I recommend Jensen USA. Jensen gives free shipping for $50 orders (smaller than anyone else), and price matches their competitors.
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Please don't confuse ebay "asking" prices with "selling" prices. Many sellers never get their ask price. some are far from it. Value is determined once an item actually SELLS. Its easy enough to check SOLD prices.