I would either buy a vit/min supplement thru a catalogue and sprinkle it over timothy pellets or, if you can get McCauley's try their M-10 balancer.
It is oat/corn/soy free and only takes one pound per day. McCauley'sŪ M10 Balancer
I started feeding the M-10 to one insulin resistant horse and my horse with serious oat/corn/soy intolerance.
It's only been a few months but so far, things seem to be going well. My food allergy horse is very calm and not looking for trouble all the time, so I know it's working for him.
The IR guy is eating the M-10 because his insulin spikes just breathing, every day, but he needs his vitamins/minerals from a safe source.