What size slot does the vendor you are buying the t-molding from say it needs
Not all same size molding uses the same size slot - so before anyone can tell you where to find the cutting blade they would need to know specifically what size blade you need (otherwise it might be too smal or too large for your specific t-molding.)
For example on t-molding.com they have this pic
Which shows the slot size as 3/32" which would be 0.0938" or 2.3813mm - so you would need a corresponding 3/32" slot cutter blade like this one : (
https://www.t-molding.com/whiteside-slot-cutter-6700a.html )
One reason no one has responded is that most US vendors do not have 15 or 18mm molding ( see attached image ) so not sure what size barb those would have ! But normally what ever vendor you buy the molding from will carry the cutter needed for their product so find the product you intend to use and check what size barb it has and then post it here so others might be able to help find the proper size cutting blade or order both the t-molding and slot cutter from the same vendor to ensure it is the correct size for their product.