Bartolini does both a vintage-style one (so it'll have a bit more character) and a more modern one (which should be more transparent) that are for guitar - boost only, just output, so no EQ. They're a bit hard to get your hands on, so you might have to reach out to them and see if they can hook you up. I actually dig the vintage-style one a lot, a buddy of mine had one in an old axe of his, and it's really nice as a lead boost or, surprisingly, for jazz - warm and fat, without killing your treble response or making it too pokey.
Other than that, the Seymour Duncan Blackouts preamp is kinda finicky in that it's always on, but if you're hooking it up to its own dedicated pot for volume control, you could switch to it (though you'd need a DP4T switch to route all the cables - it needs all 4 conductors from your pickups to work properly. It's really good for riffs, has a bit more attack and aggression compared to an all-passive circuit with the same pickups, but doesn't give you a lot of flexibility - it's either on or off, and is more coloring than flexible like you'd traditionally find in a BCR.
If neither of those strikes your fancy, or if you want proper EQ, your best option is either building your own pedal-style preamp, cannibalizing an existing pedal, or finding a smaller pedal builder who can do that for you - if you're going that route, I recommend something that has clean gain but not a lot of clipping (if you want more control and boost ability, but not a lot of extra stuff in your signal) or something that tops out at Blues Driver/Tube Screamer levels of gain so it's a bit more manageable and doesn't require a second pot to have independent output/gain controls.
You could also dig around Reverb/Ebay to find an old BCR or Carvin preamp or something similar (sounds nice and authentic to the build, but can be a pain to wire, and the Carvin is meant to be always-on and needs 5 knobs - volume, tone, active treble, active bass) or go with a Fender midboost or something like that (though I'm not a huge fan of these because IMO they can get congested and too nasal, though for some people just having the mid push is nice).
