Need replacement chainring for Shimano Ultegra FC-6503 42t chainring

Ultegra 6500 components are a 9-speed group made in the early 2000s and maybe late 1990s. The bolt circle diameter (BCD) on all performance-oriented Shimano road cranks of the time was 130mm. Thus, any chainring with that BCD will fit.

Dura Ace 7800 was introduced in 2004, and the BCD was still 130mm. Thus, that chainring will fit. I don't know if the ramps are optimally positioned for shifting onto your big ring, but I'd assume they couldn't possibly hurt. It was only with Dura Ace 9000, released in 2012, that Shimano went to 4-arm cranks with a 110mm BCD.

There are numerous third parties that make 5-arm, 130mm BCD chainrings. Full Speed Ahead (FSA), Sugino, and Vuelta are some brands I remember that make this type of chainring and that are inexpensive (and there are probably more; this is just my memory). If you're in the US, at least some of these brands should be distributed by Quality Bicycle Products, meaning that any shop with a QBP account (i.e. almost any shop) can get their hands on that brand. I suspect FSA is, but I don't work in the industry and I can't confirm.

A chainring without the appropriate ramps will not shift as well as a stock Shimano chainring. However, you presumably need a new chainring, so you may have to make this small sacrifice if the shop Andrew Henle linked to in comments can't supply you.

I did some looking around and was surprised by the lack of options for a proper ramped and pinned 42t 130 middle. But, there's still TA, the Alize model in this case. Don't get a non-rampy chainring to use with STI. It's too much of a compromise. FSA has one too.