For panels, I've had decent results ordering from Bimble Solar — good range and they know their stuff. Renogy is another one that comes up a lot for van builds, reasonable quality and the kits are handy if you're just starting out.
For lithium batteries, Fogstar are the go-to recommendation around here for good reason. Their 12V 100Ah and 200Ah cells are solid and the price-per-kWh is hard to beat in the UK market right now. Fogstar Drift range if you want something more plug-and-play.
Victron for inverters and charge controllers — yes they're pricey but honestly for a van build where reliability actually matters I wouldn't mess about with budget alternatives. A failed inverter when you're parked up somewhere remote is no fun. Picked up my Multiplus II from Barden UK, good service and they can advise properly.
Diesel generators — I've not gone that route myself, mostly trying to stay solar-only in the van, but Machine Mart and Mathers Electric get mentioned regularly. What's your intended use case? Backup power or primary? That changes things quite a bit.
Worth also checking eBay for ex-demo or returned Victron kit — can save a fair chunk. Just make sure you're buying from a proper reseller with warranties intact.
What's the build you're planning? Knowing the power requirements would help narrow down what size/type of kit actually makes sense.