I can't find a 250 watt MH/HPS combo (help?)

I can't seem to find one anywhere

Is it impossible or something?

So that's a bulb you can stick into an HPS fixture?

And what does "Glass lens required on fixture" mean?

I may have misunderstood. Are you looking for a dual-arc bulb (hybrid)? I have not been able to find one.
If you have a mh system, or digital ballast, I would recommend the Hortilux 250w MH Blue. This spectrum comes closest to what you are looking for in the wattage you are looking for, imo. If you have an hps system, check out the Hortilux Super HPS EN. I think Hortilux provides the best bulbs available.
I have a Growzilla set-up (dual-lamp), in order to run both hps and mh. I run 400 and 250 together, using the higher wattage bulb for the appropriate grow cycle (400w mh with 250w hps during veg, switched for flowering), along with supplementary uvb exposure.

http://www.eyehortilux.com/blue.html
http://www.eyehortilux.com/superhps.html

www.htgsupply.com
www.discount-hydro.com