Supplement recommendations?

After taking my mare to the repro vet, they cultured her and it came back negative so they suspect all her issues were from a really weird, awful first heat of the year.
They said to worm her again for the butt rubbing and to also get her a fly sheet because she breaks out in hives from bugs. I am currently researching fly sheets and will be purchasing one soon.
They also recommended starting her on probiotics, vitamin E, magnesium, and thiamine. Does anyone have suggestions on brands that they like of those supplements? Thoughts on those recommendations?

For me it depends on the horse and the feeding set up. My one mare is very picky and will only eat pelleted supplements, and the only place I’ve found I can reliably get the ones she needs in pellet is Smartpak. When she was at a boarding barn I always got Smartpak for everything because then I knew how much she was getting and there was no trusting others to measure right. Now that she’s at home I still have her on it because it’s easiest. My mini who’ll eat anything just gets the powdered supplements that I can get at the local feed store and measure out myself. Whenever I get something I talk to the vet to make sure it’s covering the bases it needs to. From the sounds of it most of the Multi-Vitamin supplements should cover everything needed.

Horse Guard and Equerry’s Choice each have the ingredients you listed. Both are high quality, bio-available, cost effective, and in pellet form which can be fed separately or mixed with a bit of feed. I have used products from both these lines and really like them.

You could call Smartpak and talk to him hevet there. Doesn’t cost or obligate you in any way but you could get a second opinion on supplements and dosage