2 part question/two part answer.
Cut your own tape / just pull some masking tape out on some clean surface like metal or a counter top and use a straight edge and cut it thin, You have to burnish the edge when its on the wood not just press it with your finger. I have just used regular old fashioned brown masking tape fut i'm sure the blue stuff will work fine.
Your technique of applying the stain is important. You can't flood the area with stain using a brush. Use a fairly dry foam brush and pull from the tape out when staining. Then pray.