Lots of places make adaptors. Not hard to find a 1.5" FNPT to 1.5 triclover adaptor.
Bradford, JME Ellsworth, or Dixon sanitary should have them.
I'm assuming, of course, that your 1.5" pipe is threaded at the end? Female or Male NPT adaptors are really easy to find. You can even get hose bib (3/4" NPT) to triclamp fittings. Plumb your whole house in them!