From my understanding there’s lots of hi spec alloy rims around for similar prices to lb carbon rims.
Lb offer rims only or fully built wheels on limited hubs, novatech or hope were the hub options when I bought from them.
If you want a particular hubset then you can ship hubs to them for the build or just buy the rims and have them built up locally.
If you do go carbon, it might be worth the weight penalty to go with brass nipples rather than alloy nipples, but that’s a whole other discussion.
Spoke count is important, if in doubt just get 32 per wheel. You could go lower if your an expert level whippet who’s 9 stone wet through.