Any alternative to Radio Shack sound level meter?

Skip the meter. Play music with good bass at the volume level you care about the most. Because the perceived loudness of bass is volume dependent, while meters are not. Also the level you get with a meter is only good at whatever frequency is playing. Your ears on the other hand will balance and average it all together. Meters cannot do this. So play music, make very small adjustments, and repeat until satisfied. 

Which btw will not happen with "a" sub. The day you get four though, watch out! Bliss!

I got UMIK-1  (USB microphone)
https://www.minidsp.com/products/acoustic-measurement/umik-1

Then I downloaded Room EQ Wizard (free program):
https://www.roomeqwizard.com/

When you enter microphone serial # it downloads calibration data.  It contains two text files - dB corrections at different frequencies for directional (microphone pointing forward) and omnidirectional (microphone pointing up) modes.  

It allows to automatically sweep frequencies and record chart, but it works only if you can connect computer to your system (I use computer for playback anyway). 

I'm with Erik on this one. The are a lot of hand held dB meters on the market. I keep one on  my coffee table. I have room control and getting all that stuff out is a PITA. With the hand held I get an answer just by flipping the switch.
Using a dB meter to set up subs is difficult. You would need to impulse test very driver and graph the results. Using say white noise my not produce equal levels between the drivers. The best way to do this is turn the subs up until they are obviously too loud then start dropping the volume a little at a time until the sub just disappears. If the sub does not disappear you have a phase/time alignment problem or just bad subs.
I like my subs aggressive. I use a curve that is up 6 db at 18 Hz dropping to 0 dB at 100 Hz. This produces the air behind that kick drum strike which is very realistic. Check out a small jazz club. Brittany Howard's new album Jaime uses a bass drum without any damping what so ever. You can feel that bass drum head shake producing a glorious boom instead of a thud.   

Kijanki all that is wonderful but now you need a very high resolution way of digitally creating mirror images of the curves then EQ on top. It takes at least a 64 bit system. The best way is with a digital preamp with room control and the ability to modify curves such as the Anthem STR and Trinnov Amethyst. 

I have a PCE 353 which I used to set up a pair of Wilson Benesch Torus Infrasonic Generators with their outboard amp/crossovers.

https://www.pce-instruments.com/us/measuring-instruments/test-meters/decibel-meter-kat_162375.htm

You only need to match the sub bass drivers to your main speakers 1kHz level using an appropriate test tone found on many reasonably priced CD demo discs. 

Use your ears to fine tune to taste. It is not as difficult as many lead you to believe.

I hope this helps!