Having worked at RadioShack for years (but no longer with them), I can tell you they simply don’t have a large enough selection in most instances. Their large-value electrolytics top out at about 50WVDC, though depending on the amp’s power supply, that may be sufficient, although most high-power amps operate at around 100VDC or more. I see you’re in L. I., NY. If you don’t mind taking a bit of a drive, there is a place here in NJ called Greenbrook Electronics on Route 22 in Greenbrook, NJ. If they don’t have the value you need in stock, they can order it, and the people there are generally very knowledgeable and helpful. Ask for Dave, if he’s still there. It’s been a while since I’ve been there. Bring the amp with you. I bet they’d be happy to help you out, identify the value you need, and even give you repair tips, should you choose to attempt it yourself. And of course, I’m here for advice as well. Good luck to you.