As a little winter project I fancy building up some wheels.

Looking to keep the cost as low as i can so planning on using some existing spare parts that have been unused for over a year. I have a pair of Sram Roam 40’s which are nice an light, but too narrow. So looking to re build them with new rims. Problem is they’re 24 spoke hubs and struggling to find any MTB rims. 27.5 x 29 or 30mm would be perfect.

Any recommendations?

Forget it, your choice of 24H will be hopeless as you have found, I am afraid.

Sell them and build something from scratch

Light Bicycle will drill their carbon rims with however many holes you want, thar’s how I converted some skinny old 26” fancy Hope wheels to 30mm 27.5”.  However, if I had my time again I’d probably take cynic-al’s advice and build something new with a more normal spoke count and regular J bend spokes.  That could just be me getting old and jaded though

Personally I’d like my mtb wheels to have more than 24 spokes. I have road / gravel ish wheels with 24 and when one snapped the rim goes massively out of true. Built all my mtb ones with 32 spokes for more reliability / less stress per spoke. So I’d sell what you have and either buy new complete wheels or components to build new wheels if you fancy giving it a go (it is pretty satisfying).

I was hoping to find some cheap rims to give it a go but theres nothing about. Nothing I can afford anyway. Was hoping to spend £50 at the most.

Unlikely for mtb with 24 holes

Cheap rims, 24h, 650b. Try the JRA sale (https://www.justridingalong.com/product-category/sale/rims-clearance/) e.g. these (£30 a pair!); https://www.justridingalong.com/product/trail-650b-24h-21-black/

24h 2cross will not be good for a mountain bike wheel, specialised used to spec on their bikes 24h wheels which fell apart pretty quick. You will probably need new spokes too as ERD will probably be different. spokes will be expensive as straight pull, best to sell wheels on if any one will buy

Either sell (or swap for wider rim wheels) or keep them as a spare set with light tyres. It may be a false economy to re-build them with new rims as the hubs may be shagged. I’m not familiar with them but looking at the spec they seem like decent wheels.