I use bareMinerals loose powder foundation. These are some of my favorite brushes. There are a few different kinds of brushes in the pic. Sorry, it was from when I was washing my brushes that I took with me traveling (top pic is just washed, bottom is dry). I like all of the face brushes for my loose powder foundation. I have sensory issues. They are all super soft and great quality, IMO.
5 of the ones pictured would fit what you're asking about, including the Luxie Round Top blender mentioned by u/caraliscent. That's the Luxie 532, then the Real Techniques RT 301, RT B03 Powder Bleu complexion, bareMinerals seamless buffing brush, and RT mini buffing brush based on the standard size buffing brush from their face line.
For a mini review & comparison, in terms of density, the RT 301 and mini buffing ones are the least dense, then the RT powder bleu, then the Luxie 532, then the bareMinerals buffing. All are super soft bristles. The Luxie probably has the best combo of density and super soft feeling bristles with a luxurious feel a step above RT for quality.
I do like it a lot for my loose powder foundation. However, I have been reaching for the RT ones above the others because I find that I get a nice finish and coverage with a less dense buffing brush and my bareMinerals foundation. I don't know what foundation you're using, but the bM is a little weird for a powder foundation in that it's rather creamy in texture. With the Luxie one, you can see I have a smaller Luxie setting brush next to it that I use to set my eyes. Whereas with the RT ones, I feel like I can use the same brush for foundation and to set my eyes because they're not as dense, if that makes sense. However, for my "drier" powder foundations, the Luxie is great.
The Powder Bleu does fine for foundation, but may even be superior for blush/bronzer. At least, it's a good all-rounder for powders and I really like how soft and luxurious it feels. The RT 301 with the funky silver handle and flat top is one of my favorites for overall ease of use. If you like a more dense brushes for the best coverage and best "buff", then the Luxie or the bareMinerals seamless buffing brush are your best bet. I used to swear by the latter until I decided that I don't need quite so dense a brush for every day.
You can also check out Morphe's M439 and M444 at Ulta :)