I used a cheepie digital thermometer when I started temping, it only had one decimal point 0.0 and it worked fine. You could clearly see OV etc. The batteries didnt last very long tho so I got a new Basal 2 decimal point thermometer 0.00 and works great lasted 5x as long as the cheepie one and still going strong.
Baby4u had Basal thermometer for about $18 I think + postage
But yes as Pixie said taking your temp the same time everyday and after a good sleep is most important