Please avoid buying bootleg cartridges. Doing so encourages not homebrew development, but copyright infringement. Their hardware details are listed for people who ended up with bootleg cartridges already, as well as for documenting the history of the WonderSwan aftermarket hardware scene.
There is reason to believe that more non-bootleg reflashable cartridge solutions may be available soon. Please stay patient.
Note that any photos or videos provided here, unless otherwise specified, are not licensed under the same Creative Commons terms as the wiki. Please ask their author for permission.
Notes:
Photo credits: