You don't need to spend a fortune for a sling, especially not a rainbow. I just bought a Storch rainbow ring sling for £35, which I think it is pretty good. I do also have many didymos, KKD, Oscha wraps that cost a darn sight more.
There is a group on FB affordable baby slings for sale or swap and of course the Slings and Things FSOT/Babywearing FSOT you can pick up a used sling for quite reasonable depending on which one you want.
depending on which you want, it is often better to buy used because then it has been broken in and is soft and buttery and easier to use than a brand new one.