Werks have just announced an 'adjustable' sway bar, with a group buy which you might be able to get in on. That is the only aftermarket one I know of.
However, since you have a base Sol, you can also upgrade to GXP sway's, which are stiffer, or to GXP-ZOK (GMPP) sways which are stiffer still. The latter are available at Crate Engine Depot. The former you may be able to get cheap or free from a GXP owner who has swapped to Z0K's or Werks.