compatibility for a new cassette and chain

4600 is the previous Shimano Tiagra series (4700 is the current version).

As far as I’m aware all 10 speed cassettes have the same cog spacing, so you should be able to use any 10 speed cassette and chain. More expensive cassettes are usually lighter and maybe stiffer which makes them shift ever so slightly better. They are a wear part, I wouldn’t spend too much money on them.

Note: The current Tiagra 4700 groupset has different pull ratios for the derailleur and shifter, but that doesn’t affect the cassette spacing.

The logical place to start looking for replacements is current gen Tiagra, i.e. 4700. A quick look for 4700 (really HG-500 cassettes) shows that there is no direct replacement in 12-30; however there are Ultegra 6700 12-30 available which will be compatible, although the price is higher. I assume this is the source of your frustration.

When choosing a replacement cassette the important thing to consider is the largest sprocket and smallest sprocket size compatibility with the derailleur. The maximum sprocket sizes can be found on Shimano's website: for the mid cage (RD-4601-GS) is 32T (assuming you're not running a triple crankset), or 30T for the short cage. I am intentionally ignoring tooth capacity because that shouldn't be an issue. It is also possible to exceed these limits in some circumstances but this is a trial and error process and I couldn't recommend it.

In terms of compatibility, Shimano and SRAM cassettes and chains are interchangeable. Campagnolo cassettes will not fit your hub. Any '10 speed' chain should work with this set up barring a few outliers. Manufacturers will recommend a certain chain but this is unlikely to significantly impact performance provided the number of speeds match, although mixing Campagnolo and Shimano/SRAM is definitely not recommended.

So to answer your question, you have the option of the same effective range (thought not necessarily ratios) from the Ultegra 6700 12-30, you can replace with a Tiagra HG-500 with a max sprocket size of 32, depending on your derailleur type, or you can go with a SRAM (e.g. PG1050) or any compatible 3rd party cassette, with the obvious exception of Campagnolo. The same applies to chain selection, there are a lot of chains manufactured by, for example, KMC that will work.

In addition to the above I would recommend replacing your front chainrings at the same time. If your chain and cassette is worn, it is likely that the rings will need changing as well even if the wear is not so noticeable.