Most proprietary feeds which say they're specifically for pots and baskets are high potash, but you can check for yourself - look at the numbers listed as NPK on the back of the label somewhere - the K part should read higher than the other two numbers. Tomato food is usually high potash - the NPK on my Wilkinson's one is 6-6-12. I assume you want this type of feed for something like tomatoes or summer bedding.
P.S. I just realised I'm using the term potash - actually its potassium which K represents, often referred to as potash. Sorry if you found it confusing...

I use tomato fertilizer in summer and pure potash as a powder for top dressing fruit bushes in early spring. You can also use wood ash, as long as it is pure and doesn't have any coal ash mixed in.

You can either top dress with sulphate of potash, if your plants are in garden soil, or find a garden centre that sells chempak products....they do a high potassium fertiliser.

Sulphate of Potash is a powder form of feed that has the highest potassium content that you will get, othewise tomato feed is what I use to feed our bulb collection.