Jim Kaufman who make the Sunrise Acoustic pickup makes a nice one.
He does not list it on his website, but if you contact him he can give you the details. Session ace Michael Thompson uses them in his Strat type guitars.
Was looking into this a while back, bought a clapton mid boost kit but it wouldn't fit my guitar without routing, i opted not to go for it. also looked into the onboard pre amps that they sell at guitar fetish...
the general consensus among the techs that i spoke to was that an onboard preamp will just cause noise, and that you're better off getting a pedal or use noiseless pickups like the clapton strat does.
if you don't mind, why do you want one? I wanted one cause i wanted to be able to control the dirt on my guitar, then i descovered germanium based pedals, the java boost in particular, from fuzz to crystal clean with your guitar volume knob. it blew my mind... if you don't have a treb booster or a really good ge fuzz (i.e. sunface), i suggest you give that a whirl first, i just leave it on all the time.
I've got the Demeter Midboost installed on my Fender EJ Strat together with Suhr FL pickups. The midboost is really tasteful not too over the top, just nice. Pushed all the way it can make my neck pickup sound like a full on neck humbucker, as for the bridge pickup it can get some really nice tele type tones.
The boost is most noticeable on cleans however on dirt things can sometimes get a little overboard I have to admit.
Everything in moderation, I guess you can't have too much of a good thing.
One point however, the battery for the midboost on my strat is placed below the pickguard...I really didn't want to chop out any wood to place a battery holder. So changing batteries can be quite a hassle when the time comes, I've been running mine for about 7-8 months now with no problems at all.