I wasn't sure whether to post this here or not but I decided what the hell!
In my travels on the net I have come across AdBard. Whilst I am generally not a big fan of advertising, this mob look promising. They claim to be "The ad network for ethical computing" and have a strict policy of only advertising "products and services which exhibit a level of commitment to free software and the free software community -- proprietary software is not welcome here." They are endorsed by the Free Software Foundation.
From what I can gather you probably aren't going to get rich having their Ads on turnkeylinux.org but it might help pay the bills, or alternatively you may be able to exchange some adspace for some TKL publicity (in the form of Ads on other FOSS sites).
Perhaps worthy of consideration?
For the record: I am in no way affiliated with AdBard or their parent company. Nor have I had any personal or professional experience with them, either purchasing Ads from them or hosting their Ads. I just like their philosophy and admire their proposal. I think they could fill an advertising niche.