id say mull around charity shops, thats where i find most of my stuff.
or bargain shops, or even ebay i guess, the pyjama bottoms at least dont have to be top quality.
for the dressing gown maybe get something a bit better quality, particularly if youll actually end up using it after.
so... yeah bog standard advice, just wander in and out of shops on the high street until you find something that fits the bill.
youll probably know which local shops are best for you, i have no idea where you live and even i did... i probably still wouldnt have a clue where to direct you.
i wouldnt worry all that much about matching it to the film 100%, pyjamas, gown and towel automatically says arthur dent to me, its a well-known enough image from the hg2g ~franchise~.
by which i mean dont sweat if you cant find THE EXACT SAME THING just poke about and see what you stumble across that looks like itll work well.