You should be able to put in third party bulbs. The bulbs in mine aren't the Kubota bulbs - I put in higher wattage bulbs. Much more better light.
Kubota parts are pricey. They do encourage you to buy Kubota branded parts, oils, etc., but from what I've seen all that does is cost more $$$. Unless you are of the belief that its gotta be Kubota branded, looking for third party substitutes is a lot less expensive and just as good (or better) in most cases.
Check out filter prices, for example - you'll pay twice as much for the Kubota branded filter. I've seen the lengthy discussions on the topic of "is X better than Y" and don't mean to start that one (again) here. Just agreeing that Kubota parts are expensive ;-)...