Lamps are the scam of the industry. Almost every projector on the market requires an individually designed lamp specific to that projector. In your case, you need a Optoma BL-FU250E replacement lamp. About $400 or so for this lamp.
250W UHP is just the power rating and the technology type. 250 watts, Ultra-High Pressure. But, what you need is not a bulb, but a complete lamp assembly, which is the only way it is sold. So, you pay for it.
There are numerous places online that sell it, I would find one of the online vendors that has solid feedback and go with them.