Agree with @ScottS, it really will depend on what you're looking for. One thing along that line of questioning would be if you're just looking for an exact part number or if you're willing to look at other part numbers.
For example, chip ABCD1234 might be obsolete and really hard to get from anywhere other than eBay because it's all "old stock". But if you understand what the chip is actually doing, then buying ABCD1235 might be acceptable (or it might not, it's critical to understand what the tradeoff ares when looking for replacement parts).
Another way forward if going down the replacement path would be to simply google something along those lines, "ABCD1234 replacement". If it truly is a popular chip, it might already have been second sourced by others on the internet.