We do not sell stuff here. This is a volunteer (mostly) staffed tech support forum. Go to Radio Shack or whoever is selling that kind of gear where you live, or Amazon, etc.
Look up Kinter Class D amplifier or Fosi Class D amplifier on Amazon. I have have very good results with both of these. Just read the description carefully in case the one you select does not come with an appropriate AC/DC wall adapter (if not, just match what you need - voltage MUST match and wall adapter amperage MUST exceed the amp spec by 1.5x or more, ideally, to give the adapter plenty of headroom in case you decide to turn your project up to 11).
I personally prefer my Fosi over my Kinter amps for two reasons (although I like the Kinter just fine, don't get me wrong)
Fosi seems a little better made, it's a little heavier and that appeals to me because reasons
the Fosi I have is bluetooth capable so when you're done with the art installation, you have something that's great for a kitchen or shop stereo
I will add that I have tried amp solutions such as those from Adafruit Industries. I generally like Adafruit stuff but I was pretty disappointed with their class D amps at 20W
(I was actually looking for something similar too so any real life experience for something that works I’m interested as well as how you wired that up to your Arduino / DF Player or whatever you used )
I have the Fosi BT20A TPA3116 mini power amplifier. It drives two Kicker KS65 4 ohm speakers wired in series for 8 ohms load* in a robot used outdoors (it's plenty loud enough to hear clearly in the driveway at even half volume). I have also run it as a shop stereo prior to that installation driving a couple of old Hitachi 8 ohm 80s era home stereo speakers and it was loud enough to drive these to the point you have to shout to tell me to turn down the music.
I have not used a DF player. I bought a few Wave Shields by Adafruit when they were still available, so 3.5mm stereo minijack to RCA in the Fosi (or Kinter, same connection).
The audio source in the treasure chests is WAV files on the SD card on the Wave Shield; for the Sentry Robot, the audio source is files on a Windows 8 laptop triggered by Serial char cues to an app in Processing using the minim library there (because Windows 8 and because Processing 2 is last version I tried to use on Windows 8 laptop since the Kinect camera open source middleware by Primesense was closed sourced by Apple when they bought out Primesense to use the open source Kinect tech for the iPhone facial recogntion, afaik)
(I think. I have to pull the thing apart today as it happens since the main batteries seem to have died but I think that's maybe what I did?
EDIT - maybe not, just Googled it and the Fosi can handle 4 ohm stereo speakers so that's probably what i did since it's simpler, I'll know later today after I get off my butt and actually get to it)
Also, I just checked Amazon Canada and it seems the BT20A Pro (mine is not "pro" - I bought it a few years ago) is on sale for $127. I mean, a rabbit at the grocery store by me was $36 the other day so I think that's still good value?
According to Amazon, you can even customize the BT20A Pro with your own op amp which is something I have never heard of in such a device, if you're into that option.
Wave Shield iirc as input using files customized in Audacity in .wav format, using not greeting card speakers but probably something like some 8 ohm,2-4" general purpose type I would have taken out of something or sourced at local electronics house (Sayal in Canada if so). That was years ago and I found the sound underwhelming for the work and price. I want to say I tried with marine speakers I recall buying at Sayal because they would have been good enough and rugged enough at a good price, but I'd be lying if I said I remembered exactly now. The Adafruit amplifier I want to say was their 20W offering - TDA something or other and it just didn't satisfy my taste. I mean, it works, but lacked meat.
I forgot to mention another amp type I like to use for projects (largest treasure chest and laser maze**) - if you can thrift one, aftermarket car amplifiers work great for big sound at a low price provided your installation/project has the space (my laser maze is my whole garage and the big chest is a repurposed hope chest). Both of mine I scored for like, $20-30 or so from Value Village/Goodwill.
**Pyle PLMRA400 400W stereo driving 2 x Kicker KS69 speakers and Legacy LA590 800W stereo, driving a dual mono custom speaker I made out of a big old karaoke machine, respectively. I just checked.
@jremington edit to last: I think it's worth mentioning that some folks might well be satisfied with the amps Adafruit offers. I am typically very satisfied with Adafruit's products (like the Wave Shield); however, it's worth mentioning I think that I can't stand what I would call, wimpy sound. My projects as I think I'm sure you know are almost all Hallowe'en things and the little speaker boxes that come on typical Spirit Hallowe'en animatronics, for comparison, imo, just don't do at all. They sound wimpy and tinny. My haunt can be heard a block away if i want it to (actually, several blocks since I run a pirate radio station, too, just don't tell anyone, ok)?