In answer to your question...I would search ebay/other lighting. you should be able to find one for 30 bucks or so, but 250MH is harder to find than 400watt...you don't have to replace MH...many older growers still use and some even swear by the yield and crop they get with MH..although HPS has been proven more efficient through flowering...It is not absolutely necessary...In fact I am flowering plants right now with a 1000MH agrosun gold bulb.

Now I am not saying that hps wouldn't be better to have...I am saying if you have 2-250watt MH, then that is 500watts of light you can use, until you feel comfortable upgrading.

this is for reference:
Most of us use 1000w hps hortilux...but I have been assured by more than one hydroshop owner, that the 1000watt hortilux blue metal halide is the Baddestass lamp on the Market. Just go to Eye Hortilux and look at the spectrum graph for the horti-Blue Caompared to the rest, including the 1000watt hps horti (I have one of these also, and no lie)...you haven't seen real buds...'til you grow with a 1000watt hortilux...peace