Blue Palms and 38 Degrees does small pours of all their beers. I guess a couple of small pours would be close to a half pint (or whatever the full pour size is).
Library Alehouse does it too. A lot more beer bars in the LA area offer this option than I initially realized - the thing is, the majority of them don't actually list it on their menu as an option, so you'd never know unless you just went ahead and asked.
Phil will sell you a half-pint. Boozy ales that are sold in snifters are not offered in smaller sizes (true also for Tony's and other places).