I haven’t see any online-only sellers that I’d recommend but here are a few key points to look at when you’re thinking, “bloody hell, that’s a good price…”
All online sellers will sell pre-packed flooring, so you’ll rarely get anything longer than 2.2m – and often that’ll be 2 x 2200mm planks top and bottom and 2 layers of 800/1400 or 600/1600 in between. That’s a lot of short planks that have to go somewhere, so look at the spec closely. Every seller will include the board spec – but often people don’t look beyond the picture.
Look for microbevels. Pretty much, all engineered flooring is microbevelled. Are they microbevelled at the ends as well? Poorer quality flooring is always microbevelled at the ends to hide any height differences at end joints. If you have a combination of lists of shorter lengths, and lots of bevelled end-joins, you’ll end up with a very busy looking floor.
If you find anything you like online, post a link here and I’ll tell you what I think.