For what it's worth, we're still using the SagePay form payment module, which as DrByte says relies on the now deprecated mcrypt library, but we can install that via our WHM control panel (Module Installers->PHP PECL->mcrypt (1.0.2). If you're on shared hosting then you're probably out of luck.

I don't know what the modern alternative to the old SagePay module is, and our business owner is non technical and doesn't want to change anything about it because it's the lion's share of company turnover, so we'll probably stick with it until something breaks :)