I only ordered fish from them once. I got three clown loaches and 5 dority rainbows from them. It was my first online order ever. The reason I decided to try online was b/c I was upset with an LFS purchase of clown loaches where I lost two the next day. (That actually turned out okay because the surviving one turned out to be a female that's grown really big in the two years I had her).
I was attracted to Diver's Den because of the guarantee and maybe more importantly, I liked the quarantine procedure they use, especially when it comes to clown loaches.
I had no problems and the fish were great. I still have them except one Dority died this year (so almost two years from when I got him, but it had nothing to do with them, I'm guessing he got injured as it was some swim bladder issue).
I wouldn't hesitate to order form them again. The only issue is that their prices are what many would consider high, but you gotta factor in the quarantine procedure they do into that and the guarantee.
I've never ordered from their regular fish invetory. If I ever do get a bigger tank, I'd probably get all additional loaches from Diver's Den.
The only other place I've ordered from is Imperial Tropicals. I've ordered twice from them and had great fish from them too. I really like their rainbowfish selection and the fish turned out great. They have a youtube channel where you can see what goes on behind the scenes and the owner Mike interacts with people on the stream.
I don't see myself buying from my LFS anymore now that I've had good experiences with these two vendors.
I've also heard good things about Wetspot but I've never actually ordered from them. I imagine I will in the future if I ever upgrade this tank. Their shipping is pricy but in the past they had a Cyber Monday promo and I've seen an anniversary promo (in Sept or Oct I think). The promo significantly reduces shipping costs.
Speaking of promos, Imperial Tropicals has done some great deals on the last two Black Fridays. They do a flat discount usually,have some BOGO deals on some inventory, and then they announce hourly deals as well on Black Friday.
Liveaquaria has had some great frozen food deals in December (at least two years ago they did).