CP2018, did you try
www.fvhstamps.com.They sell lighthouse brand and have the sizes you want and have split back black or clear mounts. You may have to buy a mount cutter and cut longer strips but it also allows you to custom fit unused uncommon leftover ones.
They are in B.C. Canada, so you don't have to worry about adding duty and customs and handling fee at Post office. Especially with a 30% + dollar differential it just did not compute to savings. They use recent postage and souvenir sheets, cancel and protect the stamps with plastic, and they even pay you back 1/2 of the shipping cost including tax if you return used postage.
I have no bias toward any particular dealer and do purchase when the price is economicaly favorable. I do purchase from them and it is worth a look to compare in my opinion.
Mike