You have several choices it seems to me. Here are a few.
1. Add a new PC. Just advertize "Seeking Cleric" then no one has a complaint when he or she joins the game. If the party is out in the wilderness, so could another party, so that is not really a problem. D&D games are full of coincidences.
2. Add an NPC, just like above, this could be the sole survivor of another adventuring party (not necessarily a good sign in a healer, lol), or someone else sent out there for some reason. Monster healers like Unicorns, Celestials and others can be interesting, especially if they can come and go.
3. You can space out encoutners more so the players have a chance to rest for a few days between big fights. You might also reduce the CRs they encounter until they get a healer. A high level party without healing might think twice before fighting the small band of ogres. That is just a new challenge. It also might encourage people to take Fast Healing as a feat.
4. Encourage one or more of them to take a level of a healing class (one form of encouragement would be finding a staff of curing, or wand of cure serious wounds). We had a group who all decided to take a single level of cleric, it really helped us with the healing situation.
5. You have a rogue. Depending on his Use Magic Device skill, he might be able to use some healing items. It would add a level of uncertainty to the game that might be fun.
6. Introduce a new intelligent item that can heal, but requires certain sorts of behavior or tasks from the party. It could be a singing harp, a paladins sword, or anything really. You get some of the benefits of an NPC without having to worry about what it is doing every round. An intelligent item can add to the game without giving the players a freebie.