There aren't many 8 string acoustics out there, and the few that are out there are super expensive. Is it possible to convert a 6 string acoustic to an 8?
Buy a cheap 12 string and modify it.

Put a new nut, and make a new bridge.

Intonation is gonna be sketchy because of the scale length
2002 PRS CE22
2013 G&L ASAT Deluxe
2009 Epiphone G-400 (SH-4)
Marshall JCM2000 DSL100
Krank 1980 Jr 20watt
Krank Rev 4x12 (eminence V12)
GFS Greenie/Digitech Bad Monkey
Morley Bad Horsie 2
MXR Smart Gate
Aside from the fairly obvious problems with neck length/width, there is one major reason this would be a bad idea - bracing.

Acoustic guitars are very particularly braced based on how many strings it will have - perhaps even based on how it will be tuned, if you feel like getting uber-custom.

Going and strapping on two more strings, even if you replace the neck with something more fitting, will destroy your guitar.