Amazon.com: SiYear 3.5mm 1/8 inch TRS Plug to 6.35mm 1/4 inch Jack and 3.5mm Female to 6.35 Male Plug Gold Plated Set Audio Stereo Adapter Converter for Headphone, Microphone (4 -Pack) : Electronics
LTWFITTING 1/4 in. FIP x 1/8 in. MIP Brass Pipe Adapter Fitting (5-Pack) HF1024205 - The Home Depot
1/8" Female NPT X 1/4" Push Connect Adapter - John Guest - Bulk Reef Supply
Amazon.com: VCE 1/4" to 1/8" Adapter, 3.5mm 1/8 inch Male to 6.35mm 1/4 inch Female Audio Jack Gold Plated Adapter for Amplifiers, Guitar, Home Theater Devices, Laptop, Headphones - 8 inch : Electronics
Williams Sound 1/8" Stereo Jack to 1/8" Mono Plug Adapter ADP
BrassCraft 3/8-in x 1/2-in Compression x Mip Adapter Fitting in the Brass Fittings department at Lowes.com