Sounds like (and forgive if I'm wrong) this is a gentleman who is having difficulty obtaining a blood pressure reading, and is fairly new at performing the procedure. Therefore, my suggestion would be to practice your technique before dropping hundreds of dollars on a stethoscope right now.
All the superior acoustics in the world don't mean a thing if the desired sound is unable to be auscultated due to poor placement. You don't need a six hundred dollar Bose scope to hear blood pressure, nor do you need a Littman Cardio III. A thirty-dollar ADC will do just fine (I'm having a flashback; didn't Rid go off on this a year or so ago?).
Without being there to see what you're doing, it's difficult, at best, to suggest a solution, but I often see newer providers rushing the procedure (they think I'm grading them on speed, or something--I'm not), or placing the diaphragm in the wrong place (I tell them to use the tendon as a landmark, the artery is usually directly medial to it). If you're having difficulty in having enough room to place both the cuff and your scope, perhaps try a smaller cuff size, or place the cuff higher.
Comes down to practice. Keep at it, and you'll figure it out. Best of luck to you.
Later!
--Coop