For my Bx, I had to replace one of the marker lights in back. It was the same thing, the bulb from Kubota cost a fortune. I just went to an auto parts store and got something off the rack that looked close, cost something like $2.
HA! Found it! Man was that painful. I saw something close in the Parts Unlimited catalog (Thanks for that idea Ken!) and it was made by Eiko, so I went to Eiko's page and searched there. Found it in a list of motorcycle/snowmobile lamps. The magic number is A-3603, and here is a link to one source of them, but now that the number is known you can search on that:
A-3603 - 25/25 Watt, 13/13 Volt T-6 Recreational Vehicle Bulb | Bulbs.com
$0.69/ea plus $6 flat shipping, so I think I'll get a few...
For those that are interested in a brighter bulb than the factory, they are available. Here is one source
01-HM202 - Headlight Bulb for many Arctic Cat, Honda, Polaris, Suzuki & Yamaha ATVs & Motorcycles | ATV Parts | MFG Supply