Nowhere else to get it.
Russia drains $500-600 Billion from its enemies exporting oil and gas each year. They are just asking Ukraine to add to that.
If they can’t pay, then yes, there will be non monetary costs like having policy dictated, but if they just pay their bills there won’t be any problems.
This is a world where the spigot is more powerful than weaponry. They have the gold and they make the rules.
The problem to begin with, was trusting Russia to be a stable business partner.
Even China has cheap alternatives to the recent natural gas deal that was only agreed on after 20 years of negotiations because a cornered and desperate Putin caved on the price and made a bad deal just so he could score political points with ill informed Russians. Putin is not in the tyrannical position of power you expect everyone to bow down to.
There are alternatives for Ukraine and others in the long run.
Putin shut off gas deliveries to Ukraine in June. That card has already been played.