On the Internet, there isn’t a single complete “source of truth”. The available routes depend on where you are in the network. A looking glass server with a full Internet routing table (940k routes at this time of writing) is your best bet, some of your customer routes are probably aggregated though.
Give https://lg.he.net/ a try. You can query multiple routers. For example, here I checked IP address 188.8.131.52:
This IP address belongs to my ISP (KPN - The Netherlands). This is a large ISP that advertises a /14 network. Such a large /14 block shows up everywhere. I checked a router in Germany, the USA, and New Zealand.
Next, I looked up a /29 route in the route-views.optus.net.au looking glass server (Australia):
*> 184.108.40.206/29 220.127.116.11 0 7474 i
* 18.104.22.168 20 0 7474 i
* 22.214.171.124 10 0 7474 i
* 126.96.36.199 0 7474 i
* 188.8.131.52 1 0 7474 i
This is a small /29 network so it’s likely to get aggregated somewhere: When I look up 184.108.40.206 on lg.he.net, here’s what I see:
It shows up on these routers in Germany, the USA, and New Zealand but as a /16 route. Somewhere, it got aggregated.
In your case, you probably also have customers with small networks. Your best bet is to check multiple looking glass servers, close to your customer. There might be other websites similar to https://lg.he.net/ so you can look this up quickly on multiple routers simultaneously.
I hope this helps!