By Katie Marsal

Published: 11:00 AM EST

In a research note released to clients on Thursday, PiperJaffray said it believes that Apple's product availability is improving, particularly for both versions of the iPod Shuffle and Mac mini. The research and investment firm also noted that Mac OS X Tiger is outselling TurboTax at one of the Internet's largest online retailers.

"Over the last several weeks, the estimated time to ship every Apple product either remained the same or was reduced except for the iPod U2," senior analyst, Gene Munster, wrote in the report. "Though it might be a stretch, the increase in estimated days to ship the iPod U2 (same business day to 1-2 days) could be related to the strong U2 Vertigo 2005 Concert Tour ticket sales."

The analyst also noted that Apple products continue to be top sellers Amazon.com. In his latest checks, he found that 4 of the top 10 best selling computers on the online retailer's Web site are made by Apple. And although this figure has declined from a high of 6 out of 10 on a couple of occasions over the past two months, Apple remains tied with Sony for the largest presence in the top 10.

"We believe this shows that Apple continues to hold a significant computer market presence," the analyst said.

Munster also pointed out that 8 of the top 10 best selling MP3 players on Amazon were versions of the iPod. "This is the first time we have examined the iPod's popularity on Amazon.com, but we believe it shows that Apple continues to be the dominant producer of portable MP3 players," he said.

More significant however, Munster found that Apple's OS X 10.4 Tiger operating system software, which has not been released yet, was the number 1 seller in the Software Top Seller category.

"Though it is hard to determine how the rank translates to revenue, it is safe to say that the success of the Tiger preorder was a surprise to us, given that it is outselling the seasonally strong TurboTax software," the analyst wrote.

Amazon.com is currently offering Mac OS X 10.4 "Tiger" with a $35 rebate, lowering the cost of the software to $95.

PiperJaffray maintained its "outperform" rating on Apple with a price target of $50 a share.