Looking for a 300b integrated amplifier kit for my Avantgarde Trio (107db) horn speakers

Welcome to the Audiogon. Why build, might I ask ? Enjoy ! MrD.

Check out Audio Note Kits.

For the enjoyment of building and learning - new hobby for me

I think Audio Note is the most well known.  If you want to go really budget, however, check out the kits from Asia.
I would post this question on a DIY board.  There are a few to choose from. Google DIY threads and see what you get.  I don't build but am jealous of those that do. My favorite non Audiogon-audio threads website is Audio Asylum they have a DIY Asylum as they refer to them as, it may be all tube kit however.

🙏 thank you, this is helpful