What is a reliable source for the caffeine content in tea?

TL;DR: (1) they are estimates based on a single sample and (2) No.

In order to pre-determine the exact amount of caffeine in a cup of tea, you would have to know many factors (as Choice Organic Teas says):

• The exact tea blend and batch you're steeping;
• The storage age (and conditions) of the leaves;
• The exact steeping method, including times, temperatures, water source, and steps

While theoretically someone with a really generous research grant could create a giant grid that covered a lot of these factors, nobody (that I can find) has. And the detailed reality is even more complicated than the above list, as factors like "how old was the specific tea bush the leaves came from, and was there a drought at the time" matter.

What you read online are all estimations. It's impossible to determine the exact caffeine content of a cup of tea because:

1. Everyone applies a different leaf-to-water ratio when steeping tea.
2. The higher the temperature applied the more caffeine is extracted.
3. Leaves that consist of more buds, contain more caffeine. However, in practice those teas are often steeped at lower temperatures, resulting in less caffeine.
4. CTC teas release the most caffeine in the first steep, while with loose leaf, the caffeine extraction is more gradual.
5. More oxidized teas contain somewhat more caffeine relative to less oxidized teas. Mainly because they're more intensively process, with lower resulting water content left.

So the caffeine values per category are rough estimates, and may vary strongly in practice.