Hello! I need to travel from Interlaken to Colmar in May. What's the best site to purchase those tickets?
from the company serving where you get on the first train - in this case the SBB, the Swiss Federal Railways.
Will you have a Swiss Travel Pass or Half Fare Card while in Switzerland? If so you can get that discount and only pay what is not covered.
You will probably change trains in Basel.
The easiest way is at the station in Interlaken. There is no requirement to buy tickets for trivial trips ahead of time.
You can also buy them on the website of the Swiss Federal Railways: www.sbb.ch, but since May is aaaaaaaaages away you probably can't buy those tickets yet.