Bring purchasing in-house
The only way forward is to bring IT purchasing and decision making in-house so that control is at a more local level.
My main driving factor is to install and use robust and secure systems that at the same time are financially viable long term and initially priced correctly. This is not just for big items but right down to peripheral level including cables etc. The cost savings are not always obvious which is why research needs to be done when changing or selecting a supplier. You may have to self fund an initial item or system but this may allow you to negotiate better rates and prices moving forward and all contracts and warranties are then in your own name.
Looking to work with other similar organizations that are also up against the same issues with regard to IT costs may allow you to join forces and increase your buying power even further.