Exactly what she said - Sizzix has made lots of nice alphabets in the Sizzlits and Originals lines, and also a few in the Bigz and Decorative Strips lines as well. Not sure what search term you're using that isn't bringing up Sizzix as a result - they have been selling dies at the consumer level for at least ten years and remain a major player. Another brand to consider is QuicKutz/Lifestyle Crafts (they're the same company, but it recently had a name change).
Okay, disregard large portions of that. By Googling "Happycut" I was able to find a product description that lists what it's compatible with, which will give you a list of brand names to start searching for. The Sizzix Bigz and Originals I mentioned will NOT work with it, and even the Sizzlits will require buying an additional plate.
"Each machine is supplied with four plates, and embossing mat and can be used straight out of the box with Quickutz, Jeart, Joycraft, Marianne Designs, Nellie Snellan, Go Kreate and Spellbinders dies. If you have a collection of Provocraft (Cuttlebug) dies and embossing folders or Sizzix dies you will need the SP plate which we can supply when you buy the machine. This machine cannot be used with the thick Sizzix dies or Marianne Designs Cut and Emboss folders."
The only brands of immediately compatible dies I'm familiar with are Quickutz and Spellbinders, and I don't think Spellbinders makes alphabets.