Wrong time to buy
My ex went looking for a cheap van recently. We've split up, she's about to buy her own place, and she needs to move her stuff. Renting a van is fairly expensive, so her plan was to buy one, use it for moving in stages over a couple of weeks, then sell it again.
It turns out that with Covid restrictions ending, a lot of trades have started back out again, and that's pushed up the prices because everyone wants vans. Vans which would have gone for £500 a couple of years back are now £1500-£2000. Long story short, you simply are not going to find a cheap van right now. And if you do buy one now, whatever you get will be at least £1000 over the "proper" price.
Consider a different vehicle instead
However, what you will find fairly easily are MPVs, many of which are derived from small vans. Zafiras, Kangoos, Berlingos and all those kind of MPVs have reasonably large load bays. You could put the rear seats down; or if you're definitely going to be using it as a van then you could remove the rear seats altogether. Prices for these are still running normally, so you won't be paying over the odds for one, and its resale value won't drop off a cliff.
Of course the load bay isn't quite the same, so it really depends on your requirements. Think about whether you genuinely need all that space.
Consider a trailer
If you've got a towbar on your current car, a horsebox/trailer could be a more cost-effective option. I used to provide PA equipment. A double-axle trailer gave me more load space than a Transit van, for very much less cost and maintenance. If your application for this is something like gardening or construction, you may even find that an open trailer is more useful when it comes to loading/unloading than a van would be.