The absolute best option would be to first test your hay and check up on the highs and lows for nutrients within your area. Depending on your area, hay testing can be a tad expensive at first (mine ran about $100 CAD), but is absolutely worth it because you know what you are supplementing is beneficial and not messing up any other vitamin and mineral balances ( which can create subtle or more chronic issues long-term). In that way, you’d actually be saving money because you would not be paying for any “excess” that is not going to be functional in the horse’s diet.
The next best thing ( without hay analysis) is to understand high/low vitamins and minerals present in your area’s soil analysis reports ( usually available online). Now, this is not entirely accurate, but will give you a small picture of what to reduce or increase in your horse’s diet (within reason). Then, finding a vit/min to compliment those general values.
For premade supplements, I really like Horsetechs hay balancer, but again, only when complimentary. That vitamin/mineral supplement is a decent price for what they are offering. They also make custom supplements too.