I know that you have already tried stepping down for someone else, so excuse me if I am a bit redundant here, but I think there is more than one way to search for a new DM from the existing group:
- Saying "I'm currently loosing interest, anyone else interested in DMing for a while" is what you probably did, and it will work great if there are several interested parties (We always had one or two people interested in trying out DMing). This will, however, not work all too well if people like me are in the group. I was made DM this way:
- Asking specific players if they would like to try out DMing. I was never interested in DMing until I was asked if I would mind doing it for one or two evenings, or longer, if I liked it. I had read only a fraction of the rulebooks and had no idea how to be a good DM, except for the things I "stole" from the previous DMs, so I figured I would be just about as bad a DM as possible, but the adventure I built surprised everyone (me most of all) with it's quality and I was asked to keep going, since everyone really enjoyed it. So, even the players who would never step up on their own might DM if you ask them specifically, and they might do a really good job.
Just remind your players that you don't need to be an expert in the rules (For most RPGs at least) to DM, since a) basic rules are easy to learn and b) advanced rules are usually very interesting. Also, c) as a DM you can bend and break the rules, or make new ones, if absolutely necessary. Rule of thumb in our Group is: The word of the DM is law. You can point out that there may be rules that work a bit differently than what the DM just said, but in the end, the word of the DM counts. I have no idea how that would work in RPGs that rely more heavily on rules than "the dark eye" though.
If you still can't find a new DM after that, either follow answers from other people here, or try to bring in a new person as a DM (With the consent of your group, of course). We found some great new friends that way. As long as the person knows a bit about the rules and is interested, it's worth a shot, and if it does not work out, you can still "kick" him from the group (In a friendly manner, of course) and search for someone else. The best way to go about it is to call the first adventure a trial period for the new DM, so he/she is not upset if you tell him that he probably won't be happy with your group.