RAM needed for 27" iMac 3.6GHz i9 Radeon Pro Vega

Dear Apple Community,

I've been looking to purchase a brand new iMac 27" with Retina 5K display, 3.6GHz 8-core 9th-generation Intel Core i9 processor, 8gb of 2666MHz DDR4 memory, Radeon Pro Vega 48, and (poss) 2TB storage.

On arrival I'd add 2 additional strips of 16gb RAM, with would then total 40gb of RAM.

Normally I'd purchase RAM from Crucial, but this time around I couldn't find the above new system within their search facility. On messaging them they replied:

'Thank you for reaching out to us and showing interest to our product. However, we are sorry to inform you that, this is a Retina model and we do not have any upgrades available for Retina model as of now.'

Is it difficult to get RAM for this model of iMac? Did they misuderstand my question? Any ideas? If I can't find suitable RAM I'll have to pay Apple prices.

Kind regards -

Blubber Bear.