I bought my tester from a company called Testing Equipment based out of Houston, TX. The same model is probably available for less from some of the China-direct vendors on eBay, but I wanted a USA point of contact to scream at should something go wrong w/ the deal.

They offer(ed?) two versions, the MT150 and MT160. I bought the 150 version because it was less expensive, but maybe should have bought the 160 in hind sight. The key functional differences are (a) the 160 gives readouts to .00x" accuracy (e.g., .157") and (b) has some PC download capability that I don't need. The 150 reads to .0x" accuracy, (e.g. .16"), which is fine except on the thinner readings such as .08", so I don't know if the wall thickness is closer to .076" or .084".

The vendor offers the same testers with different probes / transducers. I purchased mine w/ the N07 model, which has a 6mm diameter face and is the smallest they offer. I contacted them directly about buying a second N07 at the same time, because I suspected I was going to have to hand-fit one of them to read the curved surface of a cylinder bore properly... and I was right. I spent some time w/ 600-grit wet-or-dry profiling the face of the probe to have a convex surface that appeared to seat properly against the inside of my 4.375" tapered ring compressor.

Although the instructions give some different methods for calibration, especially considering I was working w/ curved surfaces, I think the best approach was the one that AndyF suggested: measure the thickness at the top of two adjacent cylinders w/ a caliper, then take a reading w/ the tester at the identical spot and calibrate the tester to match that.

I don't recall the cost of the individual components, but the invoice I found for the total transaction (sonic tester kit w/ probe) was just under $400. IIRC, the 160 model is $50-60 more, which I didn't feel was worth it at the time.

I hope this helps. I've used the tester to read a couple of different 440 blocks and have been able to get consistent readings after finding the best calibration approach and some hands-on experience to get the best "feel" for positioning the probe, etc.