You're right, the new Volk TE37 G2 is currently only available in 20" but 19" and possibly 18" fitment for the M3 will be available soon.
There are other Volk Rays wheel styles that have 18" & 19" M3 fitments also, without having to use spacers, like the RE30 and CE28N, both forged and lightweight. These need to be special-ordered through any of the various Volk Rays wheels distributors/dealers (which all work through Mackin from my understanding) and it will take around 3 months to get them. The good thing about Volk wheels is that they will fit all your criteria and will offer the option of special colors/finishes at a modest extra cost. See Rays wheels website and you'll find a list of these various Rays Volk wheels that can be ordered in the 5x120 bolt pattern with the appropriate offsets front & back for the M3.
Personally, I'm looking to get an 18" set of Volk RE30 or TE37 (original version, not the G2) to use a track set. They are the logical choice for forged wheels because there is no sense in spending big $$ for more expensive forged wheels if you're going to potentially ruin them from the corrosive brake dust from race pad compound at track events.
2008 ///M3 Coupe, AW/FR ext, M-DCT (gone)