Proof that she did. English is hard sometimes, but this isn't one of them.
Proof that she
would,because we don't see it in the comic,that what it means to be off screen ya nut.
From what we see in the comic,we can quite clearly see she had gone to great lengths to get to where she needs to,so there's nothing indicating that she wouldn't/couldn't do whatever it takes to get funds,materials,and tools.
Your original statement stated she wouldn't be able to get the funds for it,and the proof of that?,she sucked off dude,so where's the proof that she wouldn't use her body to get more funds?
The other thing that can be inferred,as per literally every mention of a modern/post modern Japanese family is the sending of their precious ones to tertiary education,so saving up from that would've been a reasonable thing to expect.
Now with the son sticking his junk into a tsundere elf to activate the 11th seal of the dragon charm in another world,that college money is just sitting around,and we can assume this more to be true due to the plastic surgery she took.
Your original statements made it sound her only source of income after she stopped being a house wife was the school nurse job,what evidence do you have that she doesn't have more funds?,because we do have evidence of her willingness to do whatever and we do have speculations based off of what real Japanese people would do with their peaceful lives.
The phrase "Easier said than done" comes to mind. Look, if you have to write entire paragraphs to defend all this prep, you kind of prove the point that there was too much prep.
Just how dumb are you?,the length of the paragraphs were to simply break down why it is so easy,and that was easy for people long before the internet was even a thing.
Do you know how easy it is to make a pipe bomb?
Two end caps on a length of pipe with a hole in one,pour the gunpowder into the pipe with one cap on before closing it with the other cap,then ensure the fuse is set in place.
They're so simple that they're dangerous to the maker,needing care if one is using metal pipes,and if one doesn't actually seal it properly,it'll just leak out,but they are no means hard to do,and explanation of mine is is literally what you can see and tell after looking at an image of one for less then two minutes tops.
See how she split those bamboo stands for the crossbows?,take a stick,wedge it between the split ends far enough,then wrap foil or bits of metal to the ends,VOILA,a switch that will connect when the split ends meet.
Using a door?,EVEN EASIER!,no need to split bamboo!
The level of difficulty shown is something anyone above the age of 15 can do when they look at examples done by others online,let alone a determined mother out for blood.