I bought an inexpensive one 7 years ago and had good service through last fall. It's a plastic meter with a small diameter metal rod to insert in the soil. It is a magnometer variety, so no batteries are required. They currently sell for $10-$15 at nurseries. I have tested mine for accuracy against other measurement systems and found it was accurate to the printed lines, about 0.1 pH. As Benny points out, you need to check several locations around the plant and discard any anomalous readings from contact with a rock, "hot" piece of bark, etc.
Here's a picture of one brand, the probe is twice as long as pictured, about 10 to 12 inches.
