I have a Felder FD250 horizontal slot mortiser.
I've been using Klein (Italy) birdsmouth slot mortising bits purchased through Laguna but I'm not sure they will continue to offer them. Not on their website anymore. Just had to go through some hoops to order a set.
The bits sold by Laguna are Imperial-sized birdsmouth bits from Klein (Italy) tools sold in a set of five (1/4" - 3/4"). About $330. Sharpening does not affect size. These are pretty nice with a 4" cutting length. All work well, except the 1/4" which is too small and whips too much.
I've looked at the Felder bits but they are only two flute as opposed to four flutes on the Kleins. And I've not heard good things about the Felder bits. But again, haven't tried them.
I assume MiniMax/SCM sells slot mortising bits?
What do you use, source?
(BTW, endmills don't work well in this application, and I don't have rpm control on my machine)