1) if you really want to, you can blend different viscosities to customize. This will require math. Only do it with the same brands of oil.
2) viscosities aren't actually all that standardized. One brand's 5wt is closer to another brand's 7wt. If DVO doesn't specify a brand, then you're probably fine with the 3wt. More on that here:
http://mahonkin.com/~milktree/motorcycle/fork-oil.html
3) 2.5 weight is pretty common. Just go to a moto shop. Should be easy to find some there.
Edit: 4) a quick Google search says DVO uses motorex 2.5wt which has a real viscosity of 14.5cst. That's the same weight as Torco RSF Lite, which I *think* is what you have (rockshox rebrands Torco oil). So I think you'll be good.