Digi-Key. You can select filters to search capacitors by type, voltage rating, capacitance value and tolerance, and so on. Pay attention to the minimum order quantity, as some aren't available as individual units. Play around with their search and filters, and you will find what you need. Here's one, a tantalum capacitor by way of example, arbitrarily selected at 50V rating. There are some cheaper in other voltage ratings, around $2. Or you might have luck on Amazon or eBay.
15 uF 50V Tantalum Capacitor Example
