If you're looking for standard collet holders or solid-style endmill holders, I would recommend tooling from Accupro, Parlec, or Lyndex. I also know people that have bought Glacern holders which seem to be of decent quality for their price (budget price). I have two Glacern facemills and some of their drill chucks, which are of good quality given the price, but I haven't tried their toolholders myself.
DO NOT buy any tooling from kennametal or Technicks. The quality difference on cheaper tooling is so great that you will instantly hear it.
Let me know if you need anything, I've been working with Haas machines for 4 years and own one too. My dayjob is teaching people to use em.