The ammo shortage is frustrating for everyone. Most agencies buy ammo from wholesalers, who I believe give LE some degree of priority. But some of the smaller agencies probably rely on the local gun store for small quantities.
There are about 17,000 law enforcement agencies, and around 800,000 officers in the U.S. The emphasis the agencies place on firearms training varies greatly, from very little to frequent, intensive training. Budgets and manpower drive training emphasis. The availability of suitable facilities is a key factor. The cost of ammunition is the least expensive element of police firearms training. You take officers off the street to go to firearms training, there is a gap in coverage, or you pay somebody overtime to fill the gap. Send the officer to training on his day off, you pay him overtime for training. If you staff your agency with sufficient personnel to build in training time so that it does not impact calls for service, it means additional officer positions. (This is one of the reasons de-funding police and reducing officer staffing is such a stupid idea-it means a reduction in training) In some departments even SWAT teams have difficulty finding training time. Most states have minimum annual qualification standards the officers must meet, and for some departments it is the only in service firearms training they will receive in a year. Others are far more aggressive about training-my department qualified four times a year, far above the state standard, and officers had a monthly training ammunition allotment. I believe strongly that the more competent the officer is with the firearm, the less likely it will be drawn and used inappropriately. I Have always found it frustrating that officers do not receive more firearms training but budgets and policy always win.
Some of the worst, and some of the very best shooters I have encountered were police officers. I had guys on my SWAT Team who consistently demonstrated extraordinary feats of marksmanship, and other, 20 year plus officers, who struggled mightily each year to meet minimum qualifications. Others are superb marksmen and a good number of competitive shooters are law enforcement officers.
As previously stated, not all officers are gun guys. It is predominantly a helping profession, not a combat profession. The truth is, most officers will complete a 25 year career and never fire a shot, and the firearm is well down on the list of tools they use in terms of frequency of use. Another factor to consider is what LE firearms instructors refer to as the "Rule of 3's". That is, the vast majority of law enforcement involved shootings occur at 3 yards or less, 3 or fewer rounds are fired, and it is over in 3 seconds. You don't need to be a bullseye shooter to deal with that.