Measurement with a caliper is not required to safely handload.
So saying, it is nice to have a digital one. Lee Valley will mail order you one (hardware prices, even from Lee Valley, are generally less than from reloading equipment manufacturers) Check your local reloading supplier for a max length tool from Hornady (if I remember corectly). If you just want the exact same OAL for some reason, buy a cheap plastic vernier at the dollar store and crazy glue it at the length of one of your factory rounds. Now you can use it as a try guage.
Max OAL is governed by two general considerations, feeding through the magazine and avoiding jamming bullets into the lands. Jamming into the lands is only a major nuisance if you then reopen the breech without firing; you can leave the bullet behind and spill powder all over the place.
Jamming the bullet into the lands DOES NOT cause any great pressure spikes. A definitive test was done where pressure was measured with increasing cartridge OAL in a pressure gun. "Just touching" gave the highest average presssures, with a drop once the bullet was into the lands. The chamber pressure did not vary greater than the usual variation seen between factory rounds.
If you are interested in setting up your ammo to the same length with diffferent bullets, OAL is not the factor, ogive length is. Sinclair sells a couple of nice little tools that can be used with a caliper to get the same ogive length.