185/70-15, THE classic Porsche 911 size, is now obsolete. It's the perfect size for anyone wanting a slighly larger footprint without going low profile. Most Porsche guys have now replaced the 185/70-15 with 195/65-15 tires, which are readily available everywhere. However, IMHO, while they have an almost identical circumference and look okay on an early 911, they look unproportioned on a 544.

My suggestion is to buy a high quality 165-15 (which is an 82-series tire, NOT a 165/80-15 which is LOWER), as it is the "correct" replacement for the original 5.90-15 bias plies. It is the tire of choice for every Porsche 356 and Porsche 912 pilot. Only two marques are top notch: Michelin and Vredestein. I really would want you to check Vredestein tires, as they are superb in every way.

What you also could do is wait for a used set of 185/70-15 to come along. Once in a while they are advertised rather affordably.