Supplements tend to work out rather expensive, and few are of proven benefit for MS, so if you’re on a tight budget, I’d stick to the only one there’s strong evidence for, which is Vitamin D3. Even that is unlikely to make a big difference to how you feel - though some have claimed an improvement. The main idea is it’s supposed to afford some protection against relapses, as it’s now thought frequency and severity of relapses may be related to low Vitamin D - though obviously nothing’s that simple. Taking Vitamin D won’t mean you never get another relapse, but might mean you’ve shifted the odds a bit in your favour.
If I was in your position, and had to pick one, for most chance of doing something, at least cost, that’s the one I’d go for.
As Carole says, B12 might also be useful, as many people with MS are found to be low in it, and it’s implicated in the repair of myelin. However, if you can persuade your GP to test you, and you have a significant deficiency, you may be able to get it treated for free on the NHS, like Carole, and not have to keep forking out for pills.
The evidence for B12 is not quite as compelling as D3 - it’s more of a “might help”, so if you don’t qualify for it on the NHS, and are having to watch the pennies, I’d pick D3 out of the two.