hey i have and old road bike with 27 x 1.25 rims and i was wondering if i could put bigger tires on it, because i cant seem to find any cyclocross tires in 27 1.25 but i found a really nice pair in 27 x 1 3/8.

would they still fit or would that half of a cm be to big???

also if i cant do this can anyone suggest a reasonably priced set of cyclocross tires in my bikes size?

Typically a 27 x 1 3/8 tire will do just fine on rims made for 27 x 1 1/4 tires... Rims will handle tired for a variety of widths, as long as the bead diameter is the same.

By the way, I am assuming you meant 27 x 1 1/4 since I think all 27 inch tires were actually labeled fractionally. However, if you look at Sheldon Brown's web page on the silliness of tire sizing over the years, you will see that there are times when things like that matter... for example a tire labeled as 26 x 1.75 is NOT the same as a tire labeled as 26 x 1 3/4. https://www.sheldonbrown.com/tire_sizing.html

Also consider if there is enough frame-clearance to accommodate a bigger tire.