Web creation tool

I think the easiest thing to do is to use ASP.NET wizards to build the forms for the basic operations.

Never used it. No idea what to do with it and where to find it

To start, visit http://www.asp.net
There, you'll find everything you need to start with it.
If you dont want to start from scratch, look for something called DotNetNuke (http://www.dnnsoftware.com/). Maybe this will help you.

I think .NET, for something like this, would be overkill and if you're not familiar with object-oriented development, it could be a steep learning curve.

if you're only going to access one field in one table, you may want to just go with a MS Access database, and put it up on a network share so everyone internally can look at it. From there, you could easily set up a form directly in MS Access and tie it to your table.

if this is a path you want to explore, let us know and we can get you the info you need to get started

A third option would be to contract this out to someone here on EE by using the "Hire an Expert" feature, I'm sure someone could whip it up for you relatively quickly.

Internally people have already access to the whole Access DB. The issue osfor external people who need to find multiple informations from multiple tables by typing in just one piece of data (a PO #)

I've requested that this question be deleted for the following reason:

no practical answers provided

The issue osfor external people who need to find multiple informations from multiple tables by typing in just one piece of data (a PO #)

this is exactly the opposite of what you said in your original post. you said it's a query on a single field NOT for external use.

there were several good ideas presented to you, just because you didn't like them doesn't make them impractical. there are no "tools" to help you create a custom piece of logic for your database, that is what programming is for. The closest you'll get is the Forms Builder from within access:

https://www.youtube.com/watch?v=Rj0ZBLlbsRc
http://www.access-programmers.com/creating-a-new-form-in-design-view.aspx

see my comment for my reason

If you need to build something (as you imply on your question), but dont have the skills, I think DotNetNuke is the practical answer you are looking for: (http://www.dnnsoftware.com/).
The only knowledge you need is to make sure DNN requirements will be met on the local server you mention.

Also, make this available to external, which was not mentioned on the original question, independs of this.