I don't think it's a bubble but I don't see 996TT values going much higher, with the possible exception of the '05 S and the GTs.
If you are serious about finding a special 996TT, my advice is to spread your net as wide as possible. Use all resources you have and that means dealers, eBay and all the internet sources, local and national PCA classifieds and consignment companies. If a car meets your initial review get a PPI done on it. The possible exception to that is a dealer car and lastly, be ready to travel to see the car in person.
As discussed here, the market is up on nice examples and highly equipped cars in rare colors are a premium. Many nice cars don't even hit the commercial market. Two years ago there were 5 to 10 996TTs for sale on eBay every week. Today there are half that and that seems to carry over to the other internet sources (cars.com, auto trader, carguru, haggerty etc.).
Here are three examples of purchases that demonstrate casting the net wide makes sense to find something special and good cars can be bought reasonably:
Two years ago I bought mine after an 18 month year search. I was looking for a rare color, paint to sample car, '03 or '04 with X50 and sub 50K miles. What I found was a '03, 32K mile, white/beige, X50, PCCB, 6 spd., sport seats, color coded wheels and a small wood package. A unicorn of sorts. It was in near perfect condition and a car with a complete service history. Of all places, I found it in rural MN at a classic car shop looking for a '70/'71 Cutlass Supreme convertible for a friend. Take a look at "hookedonclassics.com" and see if it fits the profile of a dealer that would have a unique 996TT for sale. It was a consignment car and to buy it, I had to pay a premium at that time.
Today that premium price point is typical for good TTs. I routinely get inquiries to buy the car and have a standing buy offer from a gal in the local PCA. Interest in the 996TT is clearly at a peak.
Just months after buying my car an old friend called and said he was in the market to buy a 911. He was thinking 996/997C coupe in the $30-40K range. After a ride in my '03 he was immediately interested in a 996TT. We hit the local Dallas/Ft. Worth market, drove a few cars and stumbled on a new trade at a Porsche dealer. It was a higher mile car at 59K miles, but it was a two owner TX car that had a good service history, excellent paint and interior. It was also a X50 car with the PCCB option, black on black with a 90 day dealer warranty. The car was bought for $42K which I thought was a great buy and under value for a X50, PCCB car, even with near 60K miles.
A year ago a close friend got the bug after driving mine a couple times. After about a 6 month search we found a two owner FL car on eBay. It was also a low mile car (34K) with a complete service history in excellent condition. It was a base turbo, small wood package, dark green over beige, with a dealer tune and in excellent condition. That car was bought in the low $40s and I think he bought it $5-7K under value at that time.
I still look at the dealerships, the various internet sites and PCA sites weekly and it's clear that finding a unique car will take a pretty deliberate effort. If you have a budget of $45-55, I think you can find a very nice TT.
Good luck with the search!