This is really going to depend on your budget, and how badly you need that white ink option.
I called The Magic Touch for an opinion, due to the company's expertise in short-run transfer-type products of many and various kinds, and spoke to the extremely helpful Alan Norman.
If you're looking at the more affordable laser printer route there isn't currently a white option on those devices, although it is possible to get white substrates. Alternatively, as Alan pithily puts it "you get a paintbrush out and and paint in the white areas on the back of it".
That said, it's worth keeping an eye on developments in laser printing technology, as OKI has launched a new LED printer in the States that has a white option. It should be officially launched in the UK next month.
Alternatively, Mimaki and Roland both have inkjet devices with a white option although these are more expensive bits of kit. If you're going to go down this route be sure to look into the details of keeping white ink flowing, especially if you're not going to use it every day.
Third alternative, find a digital print service provider who's already invested in suitable kit and is geared up to produce short-runs including white ink.
Hope this helps, as always any further input or advice from the esteemed PrintWeek readership would be much appreciated.