Unfortunately the only 1/48 WW1 bombers I know of are the old Aurora D.H.10 and Gotha G.IV.
Both of these have been out of production for decades, and have not been re-issued by SMER or Glencoe unlike some of Aurora's other kits.
You might have some luck finding them on Ebay or at a second-hand sale, but one in good condition with original box is likely to be ridiculously expensive.
Aurora also made kits for the Breguet 14 and D.H.4, both single-engined light bombers. These are also O.O.P and much in demand by kit collectors, so are hard for modellers to find at a reasonable price.
I have the Gotha G.IV which I got for around £15, as some of the parts had been painted and a couple of bits were missing; It's probably the most expensive kit I've bought so far, in any subject or scale :P - but as a great fan of 1/48 WW1 aircraft, there isn't much choice at the minute when it comes to multi-engined bombers!
Let's hope that Roden or Eduard produce some kits of this type in the future. Their 1/72 aircraft are very impressive, I've built their Felixstowe F.2A twin-engined flying boat.
There's a much better selection in 1/72 scale, though they're not 1/48 they are big and impressive models. Airfix's Handley-Page 400, Eastern Express's ex-Frog Vickers Vimy and Glencoe's Martin MB2 are all cheap and readily available.
Roden/Toko's Gotha and Felixstowe (I think they also do a Zeppelin-Staaken bomber, not sure though) are more expensive, but well worth the money.