Since I primarily use Loksound & TCS decoders for my DCC installs and have a LokProgrammer I haven't needed to have a decoder pre-loaded with a sound file in a few years now. That said, Litchfield Station & Tony's Train Exchange are two that have done it for me in the past and are very reputable.
Of the two, I order from Litchfield because of their quick turnaround on shipment (same day or next day), great communication, and VERY reasonable S&H costs ($4 for first-class). COVID has been the bottleneck on deliveries the past year but that has generally been at my end of the journey.
For the Loksound decoders there are several "plug-in options" and each of those has a 5-digit number for differentiation. The 8-pin NMRA Lok 5 DCC is "58420" and the smaller "micro" version of that is "58810".
I would call either place when ordering to assure that you are getting the correct "prime mover" sound file you want pre-loaded onto your Lok 5 decoder. And it shouldn't cost anything extra to have the sound file pre-loaded - at least it didn't in the past. Once installed you then just choose the horn & bell you want with your F7A.
If this will be your first Lok 5 decoder I would recommend downloading the latest Lok 5 Manual (V10 - Apr 2021, at this writing) from the ESU website. It's very comprehensive and can be a bit intimidating at times. However, it contains a wealth of information on programming any Lok 5 decoder.
And, if you have any questions on programming, you can always post them here and we'll do our best to answer them. The ESU website also has a forum group, as well as Groups.io.
HTH...
Tom
https://tstage9.wixsite.com/nyc-modeling
Time...It marches on...without ever turning around to see if anyone is even keeping in step.