Walmart offers at least a couple generic insulins for $20-$25, but these are only the old R and N insulins, so they aren't as useful (as fast-acting or as slow-acting) as the modern insulin analags (like Humalog, Novolog, Lantus, etc.)
Even so, R and N insulins do not require a prescription to buy, as all the more modern insulins do. So, if you find yourself needing insulin, but without much money or without a prescription (or the ability to get one in a pinch), Walmart insulin may be a good option.
However, if you are on an insulin pump, or otherwise can't get by with R or N, and you don't have much cash or a prescription, then as a very last resort, without a prescription and without cash, you can probably get some Humalog, Fiasp, Lantus, etc. at the ER.
As a major pharmacy chain, Walmart does have most popular insulins, and they most likely honor manufacturers' Insulin Savings Cards, and possibly offer 340b discounts for prescriptions from certain health centers. So, you may be able to get cheap or even free Humalog, Lantus, etc., at Walmart, if you have a savings card, or a prescription from a 340b clinic that has a relationship with that Walmart.