Bit subjective really. Some people swear by one brand, others will slag the same brand off and say they jammed up their such and such a gun. If I were you I'd try and buy them at a local site or local shop, since it supports the business in your area and helps it stay in business. Most skirmish sites will stock something that is reliable and decent (ish), since they'll likely be putting em through their own hire guns, so it would be stupid for them to have stuff that is crap and wrecks stuff in their own shop, although it's worth noting that they are also businesses, so economy may be favoured over ultimate quality, thus it's not an absolute guarantee of using the best available when you buy from a skirmish site shop.
If however, you prefer to buy online, Zero One is not a bad place to get BBs from. They have a fairly varied stock and postage costs are not bad, with quite a few options, and they do tend to have an up to date stock level display on their website, so you're generally going to get stuff quickly. You'll get arguments from people over whether the Zero One branded BBs are any good, I know several skirmish sites which sell them and I've used them with no issues myself in GBBs, AEGs and springers, but I have seen a few people say they're not so good, so it's up to you as far as they are concerned. Either way, Zero One stock G&G PSBPs, Excels, TMs, Marushin and a few others, all of which will rarely get a complaint, so you can go with pretty much any of those and expect few issues. Most people will probably say Excels are a good choice.
If you're going for .28s, that might be a sniper rifle I'm guessing, and if that's the case, then it's not a bad idea to sift through them manually and wash them gently to get any crap off them, some bags of BBs (even the good ones on occasion) can have tiny bits of crap in the bottom of the bag, and even the very best BBs can have an occasional iffy one in there, sorting and washing them will avoid that stuff getting into your gun.