Replacement bubble seal for Capitol windows

I have MI - Capitol windows in my home - built 1997. The bulb seal on the bottom window has deteriorated on most of them so I'm looking for a place to purchase replacements. I've pulled the seal out of the "T" slot on one window to take to a local Lowes and Home Depot, but they don't carry parts except for their own windows. Unlike the 58-171 pictured above, the top of the "T" is parallel to the bottom of the window. Where can I purchase replacement bulb seals for these windows?

Thanks for your response. The width of the "T" is 1/4 inch, and the diameter of the bulb is approximately 5/16 inch.

Please help me identify/find the weatherstripping in the attached diagram. It is from a Patriot series window (Ryan Homes)and is on the bottom of the movable lower window. The weatherstripping seals between the window bottom and the lower window frame.