Hi!

I have a 2019 Trek Marlin 5, 18", 29 and want to replace the bottom bracket. My current one is
VP-BC73, 73(shell width), BC 1.37x24T, 125.5mm (square)

Unfortunately, I can't find the exact BB. Could you recommend a replacement?

IF the cranks are off, you can measure the difference between the offsets for a guide.
Here's a chart from-
https://www.sheldonbrown.com/bbsize.html

Shimano or anyone else that makes cartridge BB's will probably work. You put all the info you need in your OP. Just look for another BB maker that has the spec's you wrote.

You might still have an issue with how far down on the tapers the cranks sit, but as this is a very modern bike with the current for today type of square tapers, then you should be good. Just be a little leery if their description starts talking about it fitting old bikes.

There are two tapers, JIS and ISO. ISO is for vintage Campagnolo and possibly other obscure European cranks. Ask your bike shop for any 73x125.5 JIS BSA-threaded bottom bracket.

It looks like Shimano is replacing their BB-UN26 and BBUN-55 with a new model, BB-UN300. Unfortunately, it looks like nobody has these in stock yet. You might have to shop around and consider off-brands.

Yeah, when I looked at the manufacturer site for the VP-BC73. they just said they were made for modern square taper cranks. I didn't want to delve deeper in my search to find out specifically "modern" square taper they were.

I would think that the UN-300 probably will work.
https://www.universalcycles.com/shop...&category=6042
Supposedly they are in stock.

But the OP shouldn't take my word for it being a direct replacement. They should maybe call and ask someone there what they think.

Thank you for your suggestions. I went to a local Trek bike shop, and they told me that they could not find anything of the same size. They offered me a new Shimano crankset with an integrated spindle (I don't know what type it was). Total: $80 + labor. So I decided to invest this amount in tools and knowledge. I bought a bearing separator kit and 2 bearings. It was a fun project. It still works fine after 15 mi.

I wonder how to avoid problems with rare bike parts in the future? Should every single part be Shimano only?

https://bike.shimano.com/en-NZ/produ.../BB-UN300.html
that's the current gen shimano part and says they have 73mm in 123mm
So you would definately be able to find that part somewhere, eventually. Would your chainring rub on the frame if it was 1.25mm further inboard?
That's the obvious solution. For more waffling see below:

You'll notice that most NDS cups of square taper bottom brackets are not flanged. So while obviously not ideal, you can most square taper 68mm bottom brackets in your 73mm shell, and just screw the NDS cup in 5mm more. That puts your NDS crank in 5mm. Something your body can easily correct especially if using platform pedals or on a beater.

It's also worth noting that cranks will sit differently on tapers depending on wear so it's not even an exact measurement. Like if you had a 127.5mm BB, that would be totally fine. It's mm.

other factor is if the low limit of your derailer is able to shift the cage 1.25mm inboard.

One of the many reasons why I went to 2 piece cranksets with all the bikes I had that were cup and cone square tapers. Square tapers just have too many gotcha's when all the components are mix and match AND old stuff.

Most any BSA threaded shell will have a 2 piece crank and compatible BB for it. 2 piece cranks eliminate much of the stuff that cartridge BB's couldn't do for the myriad of possible right and wrong combination of cup and cone square taper spindles and cranks.