One Stop DIY
shop has 2 types that are their own proprietary concentrates: One Stop Flavors and One Stop Blends. Both types can be used singly in your own PG, VG, and nic base. Recommended % range is usually 5-10%. IMO, and that of many others...they're really wonderful! As mentioned above in another post, they also carry the Chef's Flavours from the UK, that are ready-to-use concentrates. But their recommended % to use (by Chef's Flavours), is 20%.
They also have 2 on-going coupon codes you can use at Checkout, for a discount on your entire order. Use either of these: ECF or JFK