The Ukrainian military is basically starving for more advanced equipments. From what is public information, it is clear that the West hasn't been supplying to the levels that would satisfy the Ukrainian's side needs (e.g. Anna Malyar recently remarked that Ukraine had only received 10% of what it needs).
So why doesn't Ukraine just buy these weapons? Now obviously, not all the weapons that Ukraine has received from the West are for free. But I imagine that the country as a whole has the resources to at least buy part of what it needs?
Perhaps the problem is not resources but rather that they cannot find adequate sellers for this kind of advanced/heavy weaponry since most (if not all) sellers should be countries, and even the sellers that are not countries are probably depended on their government's approvals for such transactions?
Or perhaps there is also a bit of propaganda going on here and their needs are actually for the most part quelled (though this appears to be very unlikely)?