You should be able to buy it from the Building-->Expansions menu. Please post a screenshot.
We have performed a hotfix today to fix the issue with the Research Expansions, where some players were able to place more Research Expansions in their town, even though they had not yet unlocked them. In case you had too many Research Expansions in your town because of the bug, you won't get a new Research Expansion to place until you have the right number again. So, to summarize: the bug has been fixed, and the counter is correct again.