Try amazon.
LOKĖT, AMG and Enchante Accessories Value Pack of Entry Ball Door Knob Lock Set (6 Pack) (Keyed Alike), Stainless Steel https://www.amazon.com/dp/B07K7V8L6W/ref=cm_sw_r_cp_apa_i_3L9oCbH3TGASW
I just searched "six door knobs keyed alike" and this was the first that popped up. I think you have options.
This is a bit of a longshot... If its the water pipes behind the furnace, you may be dealing with water hammering. I had something similar with my sprinkler system and it would vibrate (Though much much slower than yours). Water Hammering in short is caused by high water pressure in the pipes. Essentially the water gains momentum as it flows through and when it hits a right angle in the pipes the sudden change in momentum gets passed onto the pipe itself (Because the water won't compress to absorb the shock). Air in the pipes can also make this worse as air is compressible, but the water is not. Lastly, you see water hammering more in modern devices that shut off water quickly to conserve water. The sudden shut-off causes a shock to the flow of water (I had this problem with my washing machine).
Things to look into:
1) At a hardware store you can get various kinds of water pressure gauges. Ideally, put one on (for me it was my outside hose bib that was easiest to attach to), and measure the water pressure. I googled what the ideal water pressure should be for my house and went from there. Being in a 10 story apartment I couldn't tell you what PSI you should have.
2) Water Arrestors are a weird thing that exist ( Amazon Link). Essentially they act as shock absorbers for water pressure. I had to install 2 on my washing machine (again because of the high PSI). There's lots of different kinds that can be attached at various points in the plumbing.
3) Pressure Relief Valve - In the end I had to get one installed just after the main line entered my property. This regulated the water pressure for my entire house. This was also rather expensive. Again being in a 10 story apartment this may not be a viable option. But 1 and 2 should be worth investigating.
I would start by picking up one of these: https://www.amazon.com/dp/B01AKX8L0M/ref=cm_sw_r_sms_apa_i_WN93DbKD4XSXV
And possibly a socket to Edison adapter and a ground lift and make sure hot neutral aren't reversed.
I would start there. I know as a kid playing with solder in LEDs you had to make sure positive and negative were aligned correctly or it would illuminate. But that was in DC, and your house is AC... So I don't know if reversed hot/nuteral would matter, but that is where I would start.
Also, are they dimmer switches and are the bulbs dimmable?
Edit: if you're going to do DIY electrical work, be careful. It's pretty simple, but has the potential to really hurt you. Pick up a non contact voltage tester. Turn off circuits at the service panel. Watch some You Tube videos on how to splice, if you're really high speed read some of the electrical code... And if you feel out of your depth, hiring a pro. Changing outlets, plugs sockets, switches and fixtures is fairly simple and just matching colors. Messing with three way and four way switches gets a bit more complex
Just replace it with a non-overflow drain like this https://www.amazon.com/Stopper-Without-Overflow-Bathroom-Lavatory/dp/B07S5Y3HN9
Top seller on Amazon has really bad reviews. Magic brand peel and stick also has 1 star reviews.
The major issues are the gap between the two and also the curvature of the tub base. Whatever goes between will need to be highly flexible and also fill the gap. I'd really appreciate any thoughts here! Thank you!
On the subject of plungers, this one is hands down the best. Makes a great seal, never turns inside out, and the handle is aluminum. Worth every penny. https://www.amazon.com/Neiko-60166A-Patented-All-Angle-Aluminum/dp/B000MAXASM/ref=mp_s_a_1_3?ie=UTF8&qid=1545606675&sr=8-3&pi=AC_SX236_SY340_FMwebp_QL65&keywords=plunger&dpPl=1&dpID=31-XfeR3DpL&ref=plSrch
This isn’t a repair so much as a concealer, but a combination of a filler and coloring like Ram-Pro Total Furniture Repair System - 12Pc Scratch Restore & Repair Touch-up Marker Kit - Felt Tip Markers, Wax Stick Crayons will hide it well.