You don't need a specific baby one, the ones that go into the ear are a one size fits all and are really good. They also last ages which is a bonus but the plastic covers are expensive. Remember to take your child to the GP if they have a fever under 12 months, definetly if they are under 6 months.
I paid a couple of pounds for a digital under arm thermometer. DD is 17 month and had high temp few times twice was admitted to hospital twice to doctors. All my readings were exactly the same as gps. They did ask what type I had used and said they are usually accurate. In fact I think mine was free, part of a set with nasal aspirater nail clippers medicine dispenser etc...
There was alot of debate on this on my last placement. Forehead strips were out, as were mercury thermometers. The sister rang the company that made tympanic thermometers and they told her they could be used on infants. The axillia (underarm ones) were seen as inaccurate by some doctors, not all. I used one on a small baby with a temp and I found it accurate though.
The digital ones are cheap and go under the arm for little babies as they don't move arond as much. The tympanic ones are better for an older baby/toddler because they are very quick.
do not buy the mothercare own brand one - total rubbish - constantly under-read dd's temp - so much so that we ended up in hospital with me saying "her temp is fine" - it wasn't. A very angry meep took in back to mothercare and got a braun one!