I suggest you first try your Kioti dealer. They may be able to help you out.
You might also try a Hot Rod parts distributor or shop that specializes in high performance vehicles (sometimes bigger tires on a rod or customized car require the wheel spaced out to accept bigger tires). You'll want to know what the bolt circle diameter is as well as any cut outs for the hub.
If that doesn't work, you may have to make (or have made) your own.
It shouldn't be too difficult for a machine shop to make spacers. If your local high school has a Vo-Tech dept., they might help you out, as long as you provide the materials.
I suggest you have them made from Aluminum.
Now, if your wheel hubs have studs, you will probably have to get longer ones installed. If your rear wheels are held on by bolts...well, that'll be easier. Just make sure you get the right threads. More then likely, they'll be a metric thread. They could be a more specialized thread (JIS or something), but that's really unlikely.
If you use a spacer (especially if you use an aluminum spacer), make sure you use plenty on anti-seize compound on the mating surfaces. Otherwise, you may end up with corrosion.
By the way, I think you want to use the term track, rather then wheel base. The wheel base is the distance between the front and rear axles. Track is the distance between the wheels on the same axle (normally measured from wheel center to wheel center, if I recall).