There are actually several methods of 11.8 Seedminer: Seedminer is currently the cheapest and most recommended method by the community. It does require spending 2$ on a DSiware game to work. NTRBoot: NTRBoot does require buying a flashcart to work, the cheapest being about 10$ (maybe cheaper on other sites depending on the cart,) and magnet. I personally prefer NTRBoot as it not only gives a method that can never be blocked without new hardware, but also a flashcart for DS games. Hardmod: This requires additional parts and soldering, but is the most secure method if you have the skill/parts to do it.
As of writing this post, there is no free exploit for 11.8 as 11.8 patched out the exploits found in ≥11.7. These exploits are also not usable by the public due to their unfinished state.