There has been a huge crackdown on these loans because of the outrageous fees companies were charging and the high number of people who got themselves into deep financial trouble by borrowing against a tax refund that never came through.
Therefore they are much harder to find, and the ones that still exist technically are not loans against your tax refund. They're just short term personal loans exactly like you could get at a payday loan center except these are issued by companies that happen to also do tax returns.
H&R block would love to give you a loan with excessive fees AND leave you with the impression that you're required to pay them several hundred more to prepare & file your tax return in a few months. However, the loan is not technically a loan against your tax refund and there's no requirement for you to return to them for tax preparation service. You would need to repay the loan, but it would be perfectly legal for you to do your taxes yourself or at some other tax prep firm and then use the refund to pay the loan.
Remember the loan is due when the contract says its due regardless of whether or not you actually got a big tax refund. If your refund is delayed by the IRS, you still have to find a way to repay H&R Block or whoever you borrowed the money from.
These loans are a horrible idea. There are too many ways to get yourself into trouble and even if everything does go smoothly and you get your tax refund and pay it off on time, you still end up paying way too much interest.