There are dozens of stomp boxes on the market that are used for this. I have been collecting these Mooer Pedals and they are built like tanks, Have solid feel and most are well under $100. This is the Mooer boost pedal
http://www.mooeraudio.com/en/ProductInfo.asp?id=44 
You can also use a lot of overdrive pedals. My Mooer Green drive can be set up as a volume boost with just a hint of overdrive. You can also use some EQ pedals this way. I have an old MXR Pre amp pedal that I used occasionally for a boost of both electric and acoustic.
I'm in the same boat playing in an acoustic duo. I tried a volume pedal and unless they have a second control for the lower volume limit, you find yourself turning down too far all the time. I also get unbalanced when standing on one :? (So don't drink and pedal)
The boosters are a good idea and I tried that but found it a little unnatural. It was too abrupt.
My best solution is I use my saddle PU for rhythm level and then I have a small mike to one side for leads. It's great for when your doing runs and fills as I can really control my level accurately this way. We use only one monitor mix that clones the FOH so I hear what the audience hears.
I have to turn aside just a little to play directly into the mike which is usually when I'm not singing anyway. Also it has improved my overall sound as nothing beats a mike on an acoustic box. I have a Taylor Mini GS. The PU system is better than average but I am really liking having the mike after all these years of avoiding them because I move around too much. This gives me the best of both. The mike is always picking up some of the guitar but this will vary as I move around, so the PU evens it out.
We have played at festivals this way and the sound man was very happy to not have to ride solos.