I had that same frustration about sequin size and ordered bunches of different packets of sequins from Simon Says Stamps - mostly the Lucy ones that have been mentioned. Yes, those Lucy combos have lots of unusual little shapes, etc. However I discovered those weird little shapes, in the odd tones, didn't work for the cards I was making.
Now for the good news! In this search I discovered ONE packet of multisized sequins that I use over and over again because they go with everything! The magic/perfect sequin combo pack came from Simon Says Stamps and it is one of their own products. It's called Moonshine and has 3 sizes of sequins - extra large, regular size and a wee/super sweet tiny size (about 1/8"). Moonshine is a clear/ almost iridescent color, without any of the weird purple/pink/blue flashes that most of the iridescent sequins have. They are a cupped style of sequin, which is the normal shape we think of, so they are easy to apply and catch the light nicely.
Here's a link to product:
https://www.simonsaysstamp.com/produ...S?currency=USD
I know this didn't address your question about where to find multi-sized sequins, however, if you give these a try you may not be interested in loading up on a bunch of colors/shapes that you won't use.