Where I work I have final decision on IT related stuff but still have to get expenditures over a certain dollar amount approved by the CFO and on the really big stuff, approval by the CEO.
I found that even after doing a ton of research and presenting the best deal to meet the bare minimum need still didn't work every time, especially over the last year or so. Here are a few things that helped me.
For example, a while back I needed a new cable certifier as my very old one was slowly dying and was getting really hard to read. A direct replacement was over $3000, but I knew they'd never approve that. After some research and shopping I found a new brand of certifier that came with really good reviews and a price tag about 1/5th that of the brand name one.
I still wasn't convinced that the much cheaper tool would be sufficient for my needs despite all the glowing reviews. Here I got very lucky, the local electronics supply store had just received a couple of them and since I have spent a fair amount of company and personal money there in the past, I was able to borrow one for a few days to test it out.
There are other ways to check out discount versions of stuff you already know that only take a fair amount of search engine prowess and/or a lack of shyness. You can search about for multiple reviews or bug your colleagues to see if they are already using it.
Face to face meetings with the money people work best for this part, but a well written proposal or email will suffice if that isn't possible.
1. Explain the need
Chances are that the money people truly don't have a clear idea what it is you do exactly, a clear concise explanation will help build your case effectively.
2. The high dollar stuff
As in the case of the $3000 replacement, I initially presented the high dollar replacement saying that I knew it was a bit pricey.
3. The compromise
Present the one you actually want that is cheaper than the first. It looks like you are settling on a lesser item but are doing so to save the company money. What money people don't love that?
It may seem like a bit of deception to do it this way, but it is far less painful than working with nearly unusable junk, not to mention it makes you more productive and cost effective. I assume this knowledge will only be used for good.