I recently used East African Shuttles and Safaris for a transfer from Nairobi to Sweetwaters and back. It cost $300, which I thought a bit pricey, but I got there and was also picked up again, although I had to pre-pay the driver. East African Shuttles also organised accommodation at Sweetwaters for me and got me a much better rate than the Camp's own website's internet special. So it worked out fine for me, and I don't see why other travellers couldn't use the services of East African Shuttles, too. It took me a while to find someone who would just do transfers, because most operators want to sell you a safari, which is fine if you don't mind going on game drives morning, day and night for several days. But if you want to relax a bit in between you can just book a hotel and then book game drives while you are there. Just check with the hotel/camp first that they do their own game drives. It was no problem for me at Sweetwaters nor at Mara Serena, but I think it might have been a bit more tricky at Lake Nakuru's Lion Hill Lodge. However, I had booked a 3-day safari there ex Nairobi, so had my own driver/guide all the time. Cost was $850 including park entry fees ($80/night), which I thought pretty reasonable for a solo traveller. I booked this with African Spice Safaris.