Where can i buy new memory from?

I would go to Crucial US | DRAM, Solid State Drive (SSD) & Memory Upgrades

choose the model of mac and they list the modules they have tested with success with the type of mac

mind you not all macs can have their system memory upgraded, the site should list if your mac are one of those.

Crucial does not have a UK store presence.

I doubt increasing the RAM memory in an iMac (even if you can) is going to make your iMac perform better.

Which iMac do you have, model and year please, click top left of your screen and click on About This Mac.

If your iMac has a Fusion drive then it is unlikely you will ever get it to perform reasonably quickly.

Perhaps buying an external SSD drive and booting your mac off that would improve things.