I have a number of mount holes and bottle cage holes that I am not using. Overtime the hardware has fallen out and I would like to replace it. However, I can find any replacement hardware at shops online. Does anyone know of a good shop for this and what sizes I need?
As with most things in cycling, its probably a metric
M5 x 0.8 bolt in the bottle cage mounts. There's a small chance its a
M4 x 0.7 if its a fancy race bike, or an imperial thread if it is older than the 80's
Coincidentally this M5 thread is the same as a cleat bolt on most clipless shoes.
Your other mount holes are probably the same M5, though some manufacturers may use M6 for a rack/carrier mountpoint for the extra load/forces.
A M4 or even a M3 could be used on a mudguard/fender mount because they need a smaller hole and carry less weight.
Light mounts haven't been a thing for decades, so it could be anything.
Any hardware shop will have metric bolts these days. You don't want them too long if you're just blanking off a hole. Best thing to get is one of those plastic box assortments of 300 pieces, so you have plenty of spares.
Downside, they're not generally that strong, so I might buy some high-tensile bolts if fitting a rack to a bike.
They will be made of steel too, so if your bike is aluminium then get some copper assembly lube/clay. Otherwise you can use normal grease on the threads, and either way wipe off the excess.
Remember to clean and dry the hole before sealing them up.
Lastly, you might go for hot glue instead of bolts. Sitting flush in the hole hot-melt glue will be more aero and less noticeable. I've got some sticks of black hot-glue for this, have not yet had a need for them though.