Well, firstly, the term 'missing link' is just wrong. Creationists present the evolution of a species as a simple chain of events e.g. fish to amphibian, and claim that if the link between the two is 'missing' then there's no evidence. That's nonsense. To begin with, there may have been hundreds of transitory forms between fish and amphibian. Plus, this simplified view pre-supposes that every species is preserved.
The truth is that fossilisation is a very unlikely and chance event. Therefore, the fossil record, no matter how big it eventually gets, will only ever be a snapshot of the countless millions of lifeforms that existed on this planet before humans evolved. At present we have only discovered about 700 species of dinosaur. Compare that to the fact that there are over 4000 living species of mammal, 10000 species of bird and an estimated 8,000,000 species overall. Fossilisation may not have preserved the majority of species that have ever existed. Therefore, the likelihood that we will find even one unbroken evolutionary timeline in which every single transitory form is preserved is next to zero.
For example, we may never find fossils of all of the forms that have existed between the bear-like animals that re-entered the sea and their descendants, the modern whales. But we do have some of those transitory forms and modern whales undeniably do have disarticulated, atrophied and utterly useless hip bones still embedded in their flesh. The question is ... why? The physical evidence is clearly there that the ancestral whale had a pelvis. And the only reason it would need a pelvis is if it once had legs. And why would God create an animal to live in the sea but give it lungs so that it can drown? Perhaps the commonsense reason that whales have lungs is that they didn't always live in the sea. You don't need every 'missing link' to see what's strikingly obvious.
Evolution is a blind process of chance mutation where an advantageous change may get carried over into future generations and become the norm throughout a species. Consequently, you have to look at fossils with the right mindset. Animals and plants aren't 'trying' to improve. Nor are living things aiming towards some perfect design. Consequently, there are blind alleys where a species doesn't make it. The path from ancestral horse to modern horse isn't a linear progression of species getting ever bigger as some older text books erroneously show, like here:
The true picture is rather more complicated. The evolution of the horse is a story of successes and failures. It isn't a simple linear progression towards an ideal and so the simplistic idea of a chain and missing links doesn't hold up. Here's a more accurate diagram:
Secondly, as I said, some of the 'missing links' have been found. It's just that the hardcore Creation lobby simply chooses to ignore them or they simply don't know about them because they don't read Evolutionist 'propaganda' i.e. properly researched scientific papers. If you want missing links, they are there in abundance. Take Tiktaalik roseae for example. Tiktaalik was something like a crocodile and something like a fish, having features of both. Tiktaalik represents a transitional form; although it had gills rather than lungs, the fossils show clearly that it had bony 'hands' that evolved to allow it to haul itself along on land. and, when compared with the fossils of creatures like Panderichthys and Acanthostega, we can see a clear development of form from fin to foot. Not every step - if you'll pardon the pun - is there yet but there's is evidence of a slow and steady change.
We also find transitional forms in terms of animal behaviour too. As you doubtless know, hermit crabs don't have shells of their own and, instead, inhabit the discarded shells of sea snails such as whelks and winkles etc.. The earliest identifiable crab fossils are found in the Jurassic Era, some 145-200 million years ago and, at that time, one of the most prolific kinds of mollusc in the sea were the ammonites; a now extinct order of animals related to modern day nautilus and octopus. And guess what? Hermit crabs used their shells too. We have the fossils.
I enjoy intelligent debate. So I address these questions to the Creationist movement: Why do fossils exist of crabs using the shells of extinct marine animals? Surely if the world is only 5000 years old then there would still be ammonites in the sea and hermit crabs wouldn't have needed to migrate to sea snail shells. And why would God create hundreds of species of ammonite and then wipe them all out? And how? Surely 'the flood' cannot be blamed as we still have many species of crab and fish. The ammonites couldn't have drowned could they? Not like whales can.
Source: Julia Whitty at Deep Blue Home. My thanks to the lovely Dr Helen Scales for making me aware of this.