Samsung's Galaxy S6 handset is set to launch in stores on April 10th, but three of the major carriers and some other retailers have started to take pre-orders for the smartphone starting on Friday, March 27th. T-Mobile is even guaranteeing customers they will get their devices shipped to them before the official launch date if they pre-order. Not all retailers are going to be doing pre-orders for every model of the device though, and according to a user on Reddit that is the case in regards to Best Buy and the 128GB model of the Galaxy S6. Keep in mind this may have just been a localized scenario and could have been just that particular location.
Since this isn't coming from Best Buy officially this should be considered a rumor for the time being, but it's worth mentioning as some consumers may have been considering buying their devices through Best Buy. Also of note, is that according to a user in the comments, AT&T is also not taking pre-orders for the 128GB model, which may suggest that Samsung is having a supply issue with that particular device model. Again, neither Best Buy, AT&T nor Samsung has confirmed this as of yet so it's possible that it may be just these locations checked by consumers.
Upon inspecting the T-Mobile website where they have a huge banner on the front page alerting consumers to the Samsung Galaxy S6 now being available for pre-order, the 128GB model is capable of being selected as an option before heading to the checkout page. Sprint's website is the same and also allows for a selection of the 128GB model for pre-order, and as Verizon Wireless isn't starting pre-orders for the Galaxy S6 until April 1st, we weren't able to check their site for those details. Considering that some locations seem to be taking pre-orders of the device and others aren't, it's more likely that just the two above listed locations won't have stock of the 128GB model for people to pre-order, and could end up receiving stock of that model sometime after the launch. If you're interested in the 128GB model of the Galaxy S6, it might be wise to check with your local retailers on whether or not it will be available for pre-order.